Avant-garde literally means 'without barriers' and can cover banRAB from all sorts of genres and backgrounRAB. My local record store has an 'avant-garde' section which contains noise, no-wave, drone, dark arabient, freak-folk, modern free jazz and improv.
Six Organs of Admittance are found alongside Merzbow, Arab on Radar, Zorn, Sunn O))) and Kayo Dot.. although they couldn't sound more different they appeal to the same group of people through their experimentation, and have therefore been grouped as avant-garde.
its impossible to say what does and what doesnt fall into the genre though..