Here is the full list of all 20 ZOMBI achievements.
First Kill with the Crossbow
Finish Game in Chicken Mode
Survive the Ambush at the Church
Get into the Nursery
Get the XXL BOB
Kill 100 zombies
Kill 200 zombies
Kill your first Zombie with a Molotov Cocktail
Get into Buckingham Palace
Get into the Tower of London
Get the XL BOB
Reach a Survivor Score of 2000 points
Kill 50 zombies
First Headshot with a gun
Get into the Supermarket
Pick up a backpack (Bug-Out-Bag)
Make it to the Safehouse
Hack all of the CCTV junction boxes in London
Finish Game in Standard Mode
Finish Game in Survival Mode