Cats and ocelots are completely different animals now days.
And I don't think tamed cats/ocelots ever killed chickens.
According to the Wiki, untamed ocelots kill chickens and baby turtles but that hasn't been updated, it still says tamed ocelots turn into cats which is no longer true. I don't know whether ocelots can still be tamed, if so they stay ocelots and probably won't kill chickens, though I'm not certain.
Untamed cats are supposed to kill rabbits (I can verify that) and baby turtles.
According to the Wiki foxes can be semi-tamed, turned "trusting", there is no mention of that stopping them from killing chickens.
Or just wait for it to go in to kill the chickens and close the doors, maybe iron doors closed by redstone so you don't get close enough to scare it.
Or maybe better have a drop with open trapdoors to trap it automatically, the Wiki claims they are immune to fall damage so you wouldn't have to worry about that. You'd still have to be around to nametag it to stop it despawning but you wouldn't have to get close until it had been trapped.