Two ways:

PV-GRUB
-------
- Copy pv-grub.gz to your dom0 /boot.
- Copy hurd-pv-grub into /etc/xen, edit it for fixing access to your hurd /, swap, and memory.
- Copy menu.lst to your hurd /boot/grub/menu.lst
- Run xm create -c hurd-pv-grub, the grub menu should appear.

static modules
--------------
- Copy gnumach-xen-pae and hurd-modules to your dom0 /boot. If you still have a
  non-PAE hypervisor, use gnumach-xen-nonpae instead.
- Copy hurd into /etc/xen, edit it for fixing access to your hurd /, swap, and memory.
- Run xm create -c hurd, gnumach should get started.

Troubleshooting
---------------
- If xm complains about networking, it's Xen fault (or missing tools like
  bridge-utils, see Xen documentation for how to configure the network. The
  simplest way is network-nat.
- If xm complains about the console, gnumach probably just crashed, please send
  me (samuel.thibault@ens-lyon.org) the output of xm dmesg.

How to re-build
---------------

- To regenerate the GNU Mach images, use the --enable-platform=xen
configure option:

git clone http://git.savannah.gnu.org/cgit/hurd/gnumach.git
cd gnumach
./configure --enable-platform=xen && make

- To regenerate pv-grub.gz,

hg clone http://xenbits.xensource.com/xen-unstable.hg
cd xen-unstable.hg
make -C tools
XEN_TARGET_ARCH=x86_32 make -C stubdom pv-grub

the result is stubdom/mini-os-x86_32-grub/mini-os.gz

- To regenerate hurd-modules, use the build_hurd-modules script.