The principle between HID and halogen bulbs is different. Tere is a very different way of providing power to the bulb. They also require less power to light up (drawing less from your battery/chargingsystem) when used compared to halogen bulbs. Besides that, they run cooler too.
There are bulbs that look like xenon out there, that are a 1 on 1 swap tho. These fake-HID bulbs however have one drawback that made me refuse them from the get-go.
They run hotter then halogen. While not problematic in itself, this does increase the chance of the reflector in the headlight unit being damaged. Flaking and such have been known to happen. When that happens, changing the reflector isn't an option, and you need a whole new headlight unit.
I'd rather pay more for full HID's and be sure the reflector is in proper condition, then pay less for the bulbs, and being faced with a complete headlight unit swap later on down the road.
+1 to that, been there, done that, learned my lesson, went HID.
