Are you allowed to trip your oponent in kickboxing?

It depends on the rules. Under Full Contact (American) rules you're allowed to sweep the foot, but it has to be "boot to boot" contact. In Muay Thai you can throw full-powered kicks to the legs pretty much however you want, and you are allowed to throw the opponent from the clinch. Under Savate rules you can also kick the lower leg, which may have a tripping effect, depending on how the opponent is moving at the time.