1: Put the chickens in a cage, a bigger cage.
2: Tell the dog how bad it behaved when it does kill a chicken (slap it (don't hurt it) and speak madly too it. After a while it will understand it's bad. Get some liquid your dog doesn't like (really strong barbeque sauce, etc) and put it on the chickens. This might be cruel to the chickens but if it works your dog will stop and you won't need to do it again.
Aim for nr.2.