(けいび) A guard who protects innocent people:the security guy at the bank, the Secret Service.
(かんし) Sort of the opposite of 警備:to guard bad people – guards at a prison, a teacher who watches students at test time to keep them from cheating.
(みはり) To stand guard. Not limited to prison guards! If you’re having a picnic, you can say to your friend, “Guard the food from crows while I go to the car and get a fuckin’ Frisbee!” and you’d use 見張り for guard in that case!