lol this old chesnut.... yeh these are known as xenons although xenons is the incorrect term for them, although everyone calls them xenons...
you can fit xenon bulbs into our normal headlight clusters but they are not HID.
Although HID are generally regarded as better - higher light intensity etc... they come at a cost, 1. they're more expensive and 2. they are a lot heavier as they have a seperate power unit to generate the electricity needed to power the increased light beam.
Read somewhere a while back that these are illegal if not fitted correctly - MOT failure, or points if the police deem that they are dangerous.
They also require headlamp washers to be legal so that rules the 172 Cup and Trophy out.
Look into it 1st before you commit yourself cos it could be costly.