In Dungeon World a character earns XP for following his alignment. Is this XP earned every time the character follows his alignment or only at the session's end?
It is only at the end of the session.
From page 30 of the rulebook:
Acting according to your alignment and fulfilling the conditions of your alignment moves will grant you XP at the end of each session
And from page 78:
If you fulfilled that alignment at least once this session, mark XP.