It depends. If you have terrible vision, it would be better to wear contact lenses when practise kickboxing, to see what coming at you clearly, otherwise, it is meaningless to practise kickboxing with blur vision. The degree of kickboxing should not be too intense, to avoid that your contact lenses may come out.
You can wear contact lenses when you practice kickboxing. I have learned kickboxing for half a year and in the beginning I also wore contact lenses in fear that eyeglasses would dash off in the course of practicing kickboxing, especially practicing the turning kick, but I felt discomfort in the summer because of sweat dripping into eyes, so I wear glasses to practice kickboxing now. In fact, what matters is that when practicing pair exercise, eyeglasses are not available in case of being kicked, at this moment, contact lenses are available. But you need to pay attention to the ocular hygiene.
Nope, you should not do that, it is risky. It is a widely known that contact lenses or eyeglasses are not allowed to wear in intense exercises, especially on sparring sessions. The most important reason is there is unexpected incident during the process. For instance, when you are doing a kickboxing exercise with your partner, unfortunately, you get hit in the face. It is high risky for your eyes to get injury because of your contact lenses, maybe damage your retina. On the other side, kickboxing is a sports between two persons in a square place, your partner is not that far away from you, thus it would not affect your eyesight to find this person opposite to you. Additionally, they are easy to fall out while sparring, so it is really annoying and make you could not concentrate on your sports. Hence, it is not advisable for you to wear them when you are kickboxing.