Xenon Bulbs just replace your current bulbs I believe, But they're a Incandescent bulb filled with Xenon gas. Provides extremely bright, white light. Xenon bulbs are an excellent choice for long distance performance.
and a HID Kit is an innovative new style of lighting that exponentially increases the vehicles lighting capabilities over halogen lighting. HID lamps produce light by striking an electrical arc across tungsten electrodes housed inside a specially designed inner fused quartz or fused alumina tube. This tube is filled with both gas and metals. The gas aids in the starting of the lamps. Then, the metals produce the light once they are heated to a point of evaporation, forming a plasma.
Hope this helps.
Im not really into lights, But i googled and found this, Maybe someone can give you a simpler explanation, Hope this helps tho =D