There is no difference between HID and xenon - just different words for the same thing. Unless your talking only about the gas in the bulb. Some companies are making a xenon-like bulb by filling the bulb with xenon gas instead of halogen and tinting the bulb blue. Those are definetly a poor substitute for the real thing.
Thanks,Jay. The models that got the ZKW brand Xenons have an issue where the reflector bowls gets "burnt". They used a thin silver plastic coating and it turns brownish. This greatly diminishes the lighting. Your fix it to replace the bowls with newer, or there is a place that re-coats the silver plating to new.
McaQk.