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 Four of a Kind.
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 consecutive all-ins.
Win a hand when you have three of a kind comprised of three kings (of any suit).
Knock out Max after he's bought in with his special item.
Win a hand with a Straight.
Win a full game.
Win a showdown with just a high card.
Win a hand with a Full House.
Win three straight pots.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.
Win a hand with a 6 card and a 9 card of any suit.
Win 12 or more hands.
Win a pot with two or more opponents when you are all in.
Win a hand with a Flush.
Check a pat hand.
Win a hand with a Straight Flush.