XP can be earned in Single Player, Multiplayer and Pro Takedown. Pro Takedown will be the fastest mode to gain XP in since there is a reward for beating the pros.
You can gain XP in various ways:
•Dealt In - Varies (You will get XP just for being dealt in)
•Showdown Winner - 100XP (Winning the showdown)
•Got Out Free - 250XP (Did not put any money in the pot; folded early)
•Smart Fold - 500XP (Folding on a hand that you wouldn't of won, this is only awarded when the hand is run out)
•Winning Spree - 500XP (Win 2 or more games)