The short answer is that Khronos/Chronus are the same entity: the 'u' (and 'c') spellings are Latinisations of Greek Khronos, who was the god of time. As for Kronos/Cronus, he's a different entity: the leader of the titans. This short answer is to help you understand the broader traditional distinctions, but the long answer shows the short answer to be effectively false.

The longer answer is that the Greeks didn't pin down their gods in a fixed way. Some Greeks did of course, but Greek culture didn't require or involve subscription to universally recognised understandings of individual deities. Greek religion simply wasn't unified - or really a single 'religion' - in the way it is often assumed to be. I cover a lot of the details and complexities of Greek polytheism in this comment from a while back - particularly the final paragraph. It should help you navigate the topic. Khronos and Kronos certainly blended into one another at points - whether in early cosmogonies like Pherekydes of Syros, where Khronos is a primordial being; but this was a debate still ongoing throughout the Roman period, the simplest representative of this being Cicero, who argues they're the same entity (in DND 25).

Those are different Latinizations/Anglicizations of the Greek word Χρόνος. There have been various trends in transliteration over time so you'll see various Greek words represented differently in Latin letters. The culprit here, Χ (chi), is usually (and "officially" in most transliteration standards) rendered as "ch" because that's fairly indicative of the sound.

(This is the origin, just by the way, of "Xmas" for "Christmas", since the Greek word for "Christ", which sounds similar, starts with a chi, which looks like an X.)

However, chi has also changed sounds over time, and the Ancient Greek pronunciation had less "voice" to it, and that sound maps well to the English "k" so some will chose to transliterate with "k" in ancient texts or contexts.