A lot of people who are learning to kick (and some who have been doing it a long time) throw the kick out there at high speed with a relaxed leg-- this is a good start, BUT...

You need to focus your kick, and by that I mean when the kick hits its target, the muscles of the leg and the rest of the body tense, bringing everything to a sudden, definable stop. Where that stop happens determines how much stress your knee takes-- it should always happen before your leg reaches full extension. The same thing applies to punches and the elbow-- if you let it snap all the way out, you will injure yourself over time.

Your knee flexibility is a good thing, as long as you build up your supporting muscles, are careful with your footwork, and take things slow, you should be fine.