I would say that PLO8 has the highest variance, then PLO and i would say that variance in HoldEm games is higher with fixed or Pot limit, because in No Limit it is easier to make players fold weaker hands with a high bet, whereas they will call more often when your bet size is limited...
The players ability in a given game will be a major contributor to variance. The hi low games should have lower variance than hi only games, and the fixed limit should have less variance than nl variants.