... and a very good one about multiple genres in one album. I think the best genre for Nirvana is punk, but not punk as in a style of music, but punk as it was first conceived.. as a do-it-yourself attitude. Punk, in the late seventies and early eighties was not just 3 chord eighth notes strummed downwards in 4/4 time with some guy screaming about the government in the background. Punk was an attitude that you didn't need to be pretty or talented to be successful, in fact, those things were seen as constraints to creativity! You just needed the balls to put your vision onto wax. That's why at the beginning, so many different types of bands were considered punk: Ramones, Clash, Television, Talking Heads, Minutemen.
Grunge was a label put on Seattle bands at first in the early nineties because the music did not sound like Guns and Roses so it wasn't Rock. It was too slow to be considered Punk (see the above first description for what constituted "punk" as a musical style in the late eighties / early nineties). And all the band members looked dirty... Hence - "Grunge"!