Not too sure about other cars, but AFAIK, Xenon (HID) and Halogen projector headlamps require different lenses, each having a matching rib/wave pattern for the lightwave.
Many have used xenon blbs in their halogen housing/projector without problems, but it is not optimised. This is the reason why it appears to change colours as you view it from different angles. That has got something to do with wave theory, but i cant remember it now.
Non-projector headlamps (the type with reflective coating inside) were never designed for xenons. It is impossible to align the beam to project to the ground as the light is too widespread.