To build:

1. ./mkbuildtree
   NB. You can override paths to Xen sources and XenLinux sources via
       the XEN and XL environment variable.

2. make -C /path/to/kernel/source M=$PWD modules
   NB. The kernel sources here are your native kernel build tree, not
       the XenLinux sources referred to in step 1.

You get four modules, xen-platform-pci.ko, xenbus.ko, xen-vbd.ko, and
xen-vnif.ko.  Load xen-platform-pci first, then xenbus, and then
whichever of xen-vbd and xen-vnif you happen to need.