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