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