Actually, it's not revealed in the movie. We know at least 3 (Szayel, Aaroniero and Shrieker) were former Hollows & Arrancar, and kept their powers, for some reason (despite Shrieker clearly not being a Hollow anymore), but we don't know where the others got their powers from. I mean, none of them are Shinigami powers, and they don't seem to be Quincy or Fullbring powers either, so are all of them former Hollows? All we know is, they're allowed to keep their powers to make them realize how futile it is to resist.