(1/2)

This is actually a complicated question, because "was it known" presumes that there is one clear definition of what "knowledge" means, and what "it" is.

TLDR:
Did people in the streets know the government was committing crimes against humanity? Of course they did.
But did people in the streets know the Nazi regime had set out to exterminate all of Europe's Jews and "inferiors" and had already done so in the millions (e.g. by 1943), knowing all the particular locations and massacres, through a general ordered policy of mass shootings, deportations to gas chambers, etc? Then the answer is no.
As one Auschwitz survivor once told me in a lecture: The notion that millions of European Jews were knowingly transported off to their own execution is an insult to the victims, playing into the Nazi hands of a narrative of weak sheep led to slaughter. This was not the case.

The Nazi regime had, step by step, "downgraded" citizens rights of Jewish Germans systematically, beginning full scale with the "Nuremberg Racial Laws" of 1935. That, of course, was common knowledge. So was the steadily increasing daily terror towards Jewish citizens.

There was not, however, a general policy of the Nazi regime to trumpet their concrete actions to exterminate millions (while their publications contained numerous examples that nevertheless said aloud that this was the long term plan, e.g. Hitler's Mein Kampf). Had the Nazi regime announced concrete measures in advance, it would have risked internal strife, flight of the intended victims, or public resistance: they did say, in the abstract, that this group of people is "undesirable" and "should be gotten rid of", but avoided announcements like "Monday morning all these people in town X will be hauled off to be murdered in a far away location."
And there is plenty of evidence where the regime tried to shield the public from knowing the full extent and scale and specifics of what was going on. A lot of this is tied to the chronology of events; the Holocaust did not begin immediately upon Hitler's accession to power. Rather, it set in in stages, beginning in earnest in 1941, and reached its peak by late 1944. If you were in occupied Eastern Europe, where geographically speaking most of the murdering took place, you had a much higher chance of being an eye witness, but remember that communicating that knowledge during war time often meant the information wouldn't get very far even if you did try to tell people.

The public got introduced to concentration camps as "internment/prison" camps (releases from concentration camps after sentenced to a term there were indeed performed until 1939, in some rare cases even early 1940), while having no public access to them. So the public had been made used to their existence, while the conditions in them steadily worsened. Or to put it differently: a public known system of prison sites had been primed to become a tool of mass murder, with the public not fully grasping so.

The Nazi regime was for the most part intent on giving people basically a choice of either believing a government version of events, or believing the whispered street rumors (and often enough first hand evidence) of what was really happening. But rumours came a dime a dozen. Hence there was no direct announcement, ever, of a concrete planned mass murder in Nazi propaganda, but lots of lies aimed at depicting the planned victims as not deserving to live, while claiming that all these people were making up bad stories about the regime. On top of that, the large scale murdering camps (Auschwitz II etc) were all not on German soil - and therefore not directly in line of sight of the German public. But even in Eastern Europe, the death camps were usually not right in the public eye.

A good example for this case of public perception of regime vs reality of Holocaust is the Rosenstrasse women's riot in Berlin, where all husbands of (German) mixed Jewish/non-Jewish couples had been arrested to be "deported" to Eastern Europe, where their wives could not visit them (they were actually "earmarked" to be sent to death camps, but the protesters did not actually know that, only that their husbands were to be ominously "brought away"). When the "Aryan" wives of these men went into the Rosenstrasse street and publicly protested the impending deportation of their husbands, the Nazi regime backed off, leading to Propaganda Minister Goebbels' infamous quote in his diary that "this was a very ugly scene, and we should not repeat it." The Nazi regime was actually always afraid of public back lash, and needed to constantly present itself as strong and infallible to hide their weakness.

So, the picture of what "knowledge" meant was fairly mixed:
- concentration camps were in place and known about in public from the beginning in 1933; but perceived as "prisons";

- death camps (special concentration camps built specifically for murder) were not added into this political prison and forced labor system until Auschwitz-Birkenau opened in '42, and they were not publicly announced to be death camps either;
- on the other hand, it was common knowledge that people could "die" in concentration camps, but the general public had not seen these from the inside, and the regime went to some lengths at least to claim that they were "just regular" prisons, and that prisoners had died due to health issues or attempts to escape. (Cf. International Red Cross visit to Sachsenhausen Concentration Camp during the 1936 Olympic Games, or a similar IRC visit to Terezin later, where the International Red Cross delegation, much to their later shame, "certified" concentration camps as standard prisons).

(2/2)

So I guess it is fair to say that people who had their eyes open and did not believe the government issued lies could have a fairly clear grasp that this was a murderous regime that could not be trusted, that people died in their political prisons under horrible circumstances, and that there was lot more sinister stuff going on than your daily life in Nazi Germany would suggest, even if you had witnessed Nazi brownshirts beat up your Jewish neighbor in public on the street; but at the same time, the _full extent_ and scale of the mass shootings in the killing fields of Eastern Europe, the gas chambers, the crematoria, etc., was not known widely even until early after the war. In fact, the whole number of concentration camps in the system is still being revised upwards by historians as more documents and evidence comes to light.

In Sachsenhausen concentration camp (near Berlin), for instance, a crematorium was added only after an SS truck whose job it was to bring corpses to a Berlin city crematorium had an accident and the corpses rolled out onto the street (this was in 1938). After that, all concentration camps were building crematoria, as the SS argued this was more practical and allowed for higher (but covered) death rates.
By 1943 we know the Western governments' top echelons were beginning to get a clear idea of what was really going on. An Auschwitz escapee even had managed to get a (today famous) audience with President Roosevelt, whose basic reaction was that the Western general public would probably not believe this to be true even if told and the facts were clear, so monstrous was the evidence presented (including photographs). The British and American public were only informed of the reality of the camps in their weekly newsreels in movie theatres after footage of liberated camps could be shown - in the spring of 1945.

So what did "the German public" know? A generalization, here, is not possible. You'd have to look at each individual and what they personally witnessed, believed (or chose to believe) at the time, whether they collaborated, resisted, or tried to just keep their heads down. Nazi newspapers were certainly not printing the latest daily murder tolls or speak openly about the actual genocidal plans.

As for Hitler: German historian Michael Uhl uncovered a testimony of a high ranking German officer from Hitler's inner circle to Soviet officers after the war in Russian archives, in which that man stated he knew Hitler had been getting personal daily debriefs as to the "progress" on the extermination. But the source is unreliable, as it was given after the war, and possibly under duress.
We still do not have a direct first hand document from the war period that directly proves Hitler had knowledge or given orders on the subject (which is why the right wing extremist David Irving has promised to pay anyone a million $ if they can, a prize still not collected by any respected historian).

Yet this is a fallacy: First hand documents are not necessary. We are looking at a totalitarian regime where the whole ideology is based on direct command; had a secret of this scale been kept from Hitler, heads would have rolled. It is also a standard technique of totalitarian dictators to keep the head of state (the dictator) in plausible deniability, well removed from the attributability of any crimes committed by the regime; hence, his followers could always claim it's Hitlers subordinates "not behaving properly". The line "If only the Fuehrer knew" (what's going on here) "this" (whatever the regime's crime is) "would certainly be stopped".
"If only Stalin knew", similarly, was a widespread thing said by inmates in Soviet gulags as well, the assumption being Stalin did not know (and of course often uttered in irony). Of course Stalin did. Of course Hitler did, too. To assume anything else is, frankly, idiotic, and the overwhelming majority of historians who've even felt they needed to talk about this, thinks so too.

And as to the question of whether the Nazi regime or even Hitler himself discussed the Holocaust with foreign leaders: No. After the beginning of the war, Hitler did not meet foreign leaders anymore (unless allied with him), and the Holocaust only began two years into the war.