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