Quote from Minoke »120.8 specifies that while casting a spell, the card isn't revealed. But that state doesn't continue. If the spell is reversed for being an illegal action for whatever reason, you are no longer casting a spell thus it's drawn normally. So the 'while casting a spell' timeframe ends and the cards drawn by Selvala are put into players' hands as normal.
This seems like a reasonable interpretation, but are you sure? The "while casting a spell" timeframe has ended, but the card was draw during that timeframe. 120.8 doesn't say "The drawn card is kept face down while the spell is being cast" rather it says "the drawn card is kept face down until that spell becomes cast", the "while" part refers to the timing of the card drawn, not the timeframe for how long it remains face down I think.