The Xenon bulbs and HID are the same thing. HID or High Intensity Discharge lamps use an alternating High Voltage to spark across a gap within an inert gas (halogen). If you remember in science, halogens are inert gases on the right hand of the periodic table which includes gases like Argon and blah blah...
Xenon is just another kind of gas that the manufacturers fill the bulbs with to prevent the oxidation of the electrodes. The pressures of gases within the bulb, type and composition of gas, firing voltage and even frequency of alternation determine the colour of the light. The last 2 factors have a lesser effect.
If you use a normal parabolic reflection head light with HIDs, the resultant glare is much brighter for oncoming vehicles. Manufacturers now supply projection type of lenses to focus all the light to a desired direction. You can differentiate these by the slightly larger than half golf ball glass lens at the front. Technically you could design a reflector type of lamp to focus all light directly in front. The problem is that you cannot find a light source the size of a pin head and bright as day. Thus the epsiloidal reflector is the best together with the parabolic lens.
food for thought.
Cheers
