Hi,
There are 3 main differences between the 2 lines:
1) The country of manufacture of the actual components.
2) The Xtreme kit comes with a relay harness that will power the ballasts directly from the battery whereas the Volt kit does not.
3) The Xtreme bulbs use German made Philips anti-uv quartz glass whereas the volt kit does not.
Please view this link for more information about the 2 kits:
http://www.kbcarstuff.com/HID_Kits_H...2001_s/216.htm
Steve