Heavy metal spent a lot of time emerging, which makes it a little difficult to answer this question. In a previous answer, I discussed the emergence of the term 'heavy metal', which dates to 1970 or so in a musical context. However, 'heavy metal' wasn't really used as a genre term at the time; it was more a sonic descriptor, in the way that 'angular' is a sonic descriptor (over)used by rock critics.

Bands from the early 1970s like Black Sabbath - which we now might called 'proto-metal' or maybe even just 'heavy metal' these days - were largely called things like 'downer rock' at the time. Or 'hard rock'. Or just 'rock'. In contrast, it's perhaps with the rise of the British New Wave Of Heavy Metal movement - most famously associated with bands like Judas Priest and Iron Maiden in the mid-to-late 1970s - that you start to see 'heavy metal' consistently used not just by critics to describe sound but to refer to a genuine genre with its own aesthetic and fans.

As to the 'downer rock' of the early 1970s, a 1971 Lester Bangs review of Black Sabbath's Master Of Reality in Rolling Stone gives a flavour of how it was taken at the time. Bangs compares and contrasts Black Sabbath with Grand Funk Railroad, another loud band of the era who were an American band who--well, for more information about Grand Funk Railroad, consult your school library--but suffice to say that they were a loud band who had fewer traces of the blues than Cream and Hendrix, but who weren't quite downers like Sabbath. For Bangs:

Grand Funk's vision is one of universal brotherhood (as when they have spoken of taking their millions to the White House with a list of demands), but Black Sabbath's, until Master of Reality anyway, has concentrated relentlessly on the self-immolating underside of all the beatific Let's Get Together platitudes of the counter culture.

In the review Bangs, despite having a claim to inventing the term 'heavy metal', doesn't use the term; instead he mostly just calls both bands 'rock'. These days, Grand Funk are rarely considered 'proto-metal' - they're more just 'hard rock'. But Black Sabbath - who like Grand Funk were generally seen as 'hard rock' in the Ozzy Osbourne era in the early-to-mid 1970s - are usually seen as 'metal' in some way by modern audiences. When Bangs says that they're "concentrating relentlessly on the self-immolating underside of all the beatific Let's Get Together platitudes of the counter culture", he's indeed saying that Black Sabbath was a conscious reaction against the hippie movement. The relentlessness of Black Sabbath that Bangs identifies has a lot to do with the reasons that we associate them with metal today, where we see today Grand Funk as hard rock, but not quite metal.

The 1970s was a point when hippie culture was well and truly mainstreamed, with successful hippie groups like Crosby Stills Nash and Young doing arena tours (here playing 'Almost Cut My Hair' in their 1974 arena tour - I think the video was probably recorded at Wembley Stadium). And you did have critics like Lester Bangs loudly pooh-poohing hippie stuff as tired and out-of-date, and instead advocating music that Bangs argued reflected the death of the hippie dream. For Bangs in 1971, neither 'punk' or 'metal' yet existed, but what did exist was rock music that was nihilistic, not particularly melodic, and meant to be loud. Thus Bangs makes mentions of the MC5 and The Stooges - who are these days seen as 'proto-punk' - in relation to a Black Sabbath review - who we'd see more as metal. In 1971, in other words, there was no 'punk' and no 'metal' yet; there was just loud rock'n'roll music that was more relentless and nihilistic than usual.

There's a 1974 article by Jon Tiven in Circus Pages titled 'Black Sabbath and Deep Purple: Who Really Are The Kings Of Heavy Metal?', which compares Black Sabbath and Deep Purple at length (and more offhandedly, Led Zeppelin), and which suggests that Blue Oyster Cult and Kiss might be upcoming American competition for their crowns. Tiven is definitely using the term in a way that comports with our modern understanding, but there's still some fluidity of usage, and it's not quite understood as a distinct genre yet:

Recently Sabbath was voted Best of the Punk Rock Bands in a British publication. "Does that mean they think we're rubbish?" queried a slightly paranoid Iommi.

1974, mind you, is before the Ramones or the Sex Pistols - where Black Sabbath are called a 'punk rock band', it's because the way the likes of Lester Bangs conceptualised loud, heavy rock music was influential at this point. For Bangs and his ilk, 'punk' in 1974 meant a certain nihilism and willingness to dare to be a bit stupid and loud (in an era of twiddly, overtly-classically influenced progressive rock music). Metal takes itself quite seriously sometimes, and so people sometimes see Bangs' descriptions of the music as insulting, but he was pushing this stuff in the music as being a good thing compared to the inexplicably popular, pretentious Rick Wakeman concept albums of the era.

As to the New Wave Of British Heavy Metal (NWOBHM) of the late 1970s - a term associated with Iron Maiden, Judas Priest, and so forth - it's about the point where, in English, you start to see heavy metal as a distinct genre. NWOBHM is not really anti-hippie, per se; by the late 1970s, hippie culture had receded somewhat in the UK. Instead, the popular things to react against in the UK, musically, were instead the seeming omnipresence of disco (which I discuss here albeit in a more American context), the relative blandness of a lot of the pop of the era as would have been seen on Top of the Pops, and the headscratching pretentiousness of the (late period) progressive rock of the era. Usually, in music history discourse, this is all the stuff that punk was seen as reacting to - there was something of a feeling that the music on Top Of The Pops didn't reflect the times, politically or culturally, where it had reflected the times in 1966. And because the late 1970s in the UK was a period of social disharmony in a variety of ways, it's not entirely surprising that music that made a virtue of sounding unpretty - punk, for example - was the music that people thought mostly reflected the times.

And, in the mid-to-late 1970s, all the same factors that apply to punk also apply to metal. With the addition that, in a way, NWOBHM was also responding to the cultural capital of punk amongst the music cognoscenti, and the way that punk dominated headlines and discussions of music in the cool London press. There was an art-school background to punk that was very deliberately absent in NWOBHM - which instead took visual reference in its cover art from pulp science fiction and fantasy book covers and the like. In the punk world, the bigger stars of punk quickly graduated from making loud guitar-heavy rackets to making more atonal, spiky rackets - for instance, Johnny Rotten going from 'Anarchy In The UK' to changing his name back to John Lydon and doing stuff like 'Death Disco' by 1978/1979. In contrast to the art-school backgrounds of a lot of punk bands, the stereotype of the NWOBHM band is that they came from bleak, no-nonsense industrial towns like Birmingham and that their music reflects that (though Bruce Dickinson of Iron Maiden went to a posh boarding school and Johnny Rotten himself was very working-class in background - it's not a simple class divide). But there was certainly a perception that metal was much more popular in much of regional Britain.

Of course, the emergence of heavy metal as a mainstream-ish genre in the USA is a different story - I explain the rise of hair metal in the 1980s here, and globally is a different story altogether I suspect (but that's a story someone else will have to tell you!)

(Edit: I originally wrote this living after midnight, which is clearly the most metal time of the day, but not necessarily conductive to clear thought! I've just spent a little more time massaging it into shape)