Rulings

Selvala's parley ability is a mana ability. It doesn't use the stack and can't be responded to.
If you activate Selvala's ability while casting a spell, and you discover you can't produce enough mana to pay that spell's costs, the spell is reversed. The spell returns to whatever zone you were casting it from. You may reverse other mana abilities you activated while casting the spell, but Selvala's ability can't be reversed. You'll still have whatever mana that ability produced, and each player will have drawn a card.
Except in some very rare cases, the card each player draws will be the card revealed from the top of their library.