In a halogen headlamp light is generated by heating up a wire. The small amount of a halogen gas in the bulb lets the filament work at higher temperatures than traditional filament lamps, producing more light and a better colour.
XENON High Intensity Discharge (HID) headlamps create light by passing electricity through a mixture of gases, including xenon.
Light emitting diodes (LEDs) are solid-state (semiconductor) lamps. The one headlamp includes both the low and high beam.