Adepts have Channeling 1, which gives a 20% chance to gain xp each turn (although other things, like current xp, can modify this. Also, only unit with <bFreeXP>1 get the free xp, but all priests and arcane units have this tag.)
Priests start with Chaneling2, which gives a 30% chance to gain xp each turn (with the save caveats)
Mages have both channeling 1 and 2, so they have a 50% chance of gaining xp each turn (with the same caveats)
High Priests have channeling 2, and 3 (which gives a 40% chance of free xp per turn), so they have a 70% chance of gaining xp each turn (with the same caveats)
Archmages have channeling 1, 2, and 3 (which gives a 40% chance of free xp per turn), so they have a 90% chance of gaining xp each turn (with the same caveats)