To build:

1. ./mkbuildtree [<arch>]
   NB. You can override paths to Xen sources and a (stub) XenLinux
       build tree via the XEN and XL environment variable.

2. make -C /path/to/kernel/build M=$PWD modules
   NB. This is your native kernel build tree (or a distro provided
       stub), not the XenLinux sources referred to in step 1.
   NB. If you are cross compiling, you need to set ARCH and
       CROSS_COMPILE too.

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