Do not use pva for heavens sake, that just puts a skin on the tiles, which aren't absorbent, you end up tiling on a skin of pva, pva is to prevent fast absorption, so saying use pva then wait 48 hrs to grout is a contradiction in terms, yes you can tile over tiles, done it on many occasions, never had a problem, as with any substrate, it's got to be sound, and things to take in to consideration.