Here is the full list of all 20 Poker Night at the Inventory achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Win a game on hard.
Win a hand with a Straight Flush.
Win a hand when you have three of a kind comprised of three kings (of any suit).
Win a hand with Four of a Kind.
Check a pat hand.
Win consecutive all-ins.
Win 12 or more hands.
Win a hand with a 6 card and a 9 card of any suit.
Win a hand with a Flush.
Knock out Max after he's bought in with his special item.
Win a hand with a Full House.
Win a full game.
Knock out The Heavy after he's bought in with his special item.
Knock out Strong Bad after he's bought in with his special item.
Knock out Tycho after he's bought in with his special item.
Win a hand with a Straight.
Win a pot with two or more opponents when you are all in.
Win a showdown with just a high card.
Win three straight pots.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.