Pyrrhic Revival can cause creatures to be put onto the battlefield that have 0 toughness due to the -1/-1 counter from this effect. Those creatures are put right back into their owners' graveyards as a state-based action. No spells or abilities that would remove counters or otherwise affect those creatures can be cast or activated before that happens. Still, the creatures did momentarily pop onto the battlefield, so any relevant triggered abilities (including their own "enters" or leaves-the-battlefield abilities) will trigger. They'll be put on the stack after state-based actions are checked.