The signing made by Iain Dowie, has a year left on his City contract but does not feature in Chris Coleman's plans, despite being the second highest scorer for City Reserves last season.
Simmonds had a pre-season trial with Killie, who have now opted to take the striker until December 31st 2008.
Said Kilmarnock boss Jim Jeffries to the Daily Record: "Donovan's extremely quick and looks a real prospect, so we decided to get him in on loan. "He gives me another attacking option."
Elsewhere, former City mdifielder Paul Telfer has joined Leeds United on a short term contract.