Players that kill multiple enemies get items. These items get better with more enemies killed, eventually looping and giving the same items. Players with hardline require one less kill for killstreak rewards.

Survivor Killstreaks

Kills Reward
2 Melon
3 Apple
4 Cooked Chicken
5 Cooked Porkchop
6 Golden Apple
Sticky Grenade
Flash Grenade
9 Potion of Strength
10 Splash Potion of Poison
11-24 Repeats everything from 1 - 10
25 Diamond Sword

Zombie Killstreaks

Mobs continue to get killstreaks even if killed. For example, if a zombie kills one survivor and then dies, the next time they kill a survivor, they will get two mushroom stew rather than another porkchop.

Kills Reward
1 PotatoCooked Porkchop
2 Potato2 Mushroom Stew
3 Potato373.2
4 PotatoPotion of Strength