Here we have an intro to Macintosh boot:

http://macos9lives.com/smforum/index.php?topic=1965.msg9990#msg9990

The New World ROM also sets the "compatible" property of the root node to "MacRISC2" (machines that can boot classic Mac OS using "Mac OS ROM") or "MacRISC3" (machines that can only boot OS X or another Unix-like system).

So after the machine "basic" boot procedure (Open Firmware), we have to to enable a Mac Os 9 ROM to boot on a MacRisc3 Mac (the xServe, or any other Os X only G4). Here we have a guide on how to solve this:

http://macos9lives.com/smforum/index.php?topic=1967.msg9998#msg9998