A neat little script to quickly spinup a Xen PV Gentoo guest (domU).

pvm-xen.sh

#!/bin/bash

if [ -z "$1" ]; then
    echo "Please provide a name for the new vm"
    exit 1
fi

export NAME="$1"
export UU=$(uuidgen)
echo $NAME $UU

echo "Creating control file"
cat << XENU >> /etc/xen/$NAME
kernel = "/usr/lib/xen/boot/pv-grub-x86_64.gz"
extra = '(hd0)/boot/grub/grub.conf'
memory = 512
maxmem = 2048
name = "$NAME"
uuid = "$UU"
cpus = ""
vcpus = 2
vif = [ 'mac=00:00:00:$(echo $UU|tail -c 7|head -c 2):$(echo $UU|tail -c 7|head -c 4|tail -c 2):$(echo $UU|tail -c 7|tail -c 3),bridge=br0' ]
disk = [ 'phy:/dev/vg/$NAME,xvda1,w' ]
XENU

echo "Creating lvm volume"
lvcreate -n $NAME vg -L5G

echo "Formatting volume"
mkfs.ext3 /dev/vg/$NAME

echo "Mounting filesystem"
mkdir -p /mnt/xenu
mount /dev/vg/$NAME /mnt/xenu

echo "Extracting tarball"
pushd /mnt/xenu
cleanup () {
    popd 2>&1 > /dev/null
    umount /mnt/xenu
    exit 1
}

trap 'cleanup' EXIT
du -sh /gentu-x86_64.tar.gz || cleanup
if [ -e "$(which pipebench 2> /dev/null)" ]; then
    pipebench < /gentu-x86_64.tar.gz | tar xzpf -
else
    tar xzvpf /gentu-x86_64.tar.gz
fi
echo hostname=\"$NAME\" > etc/conf.d/hostname

echo "$NAME vm is now ready"
echo "To run the VM, type"
echo "  # xl create -c /etc/xen/$NAME"

A few notes about my local setup.
1/. I use LVM. The VG is called 'vg'. That's hardcoded at line 27.
2/. I have a handly stage4 tarball at /gentu-x86_64.tar.gz. It contains
a) /boot/grub/grub.conf (but not the rest of grub)
b) /boot/{vmlinuz,initramfs} for pv-grub. The initramfs isn't used that often.
c) DDNS is available on my network, hence line 51 is crucial.
d) PAM/Kerberos is available on my network, no passwords are stored in the tarball.
e) sshd is available, but no ssh keys. Those are generated on first boot. Security is not hardcoded (see 2d), but derived from the network. The only rescue mechanisms are initramfs shell, or offline mount the PV (see 3)
3/. The entire PV is created as an ext3 volume. ext4 is broken in the current version of pv-grub. Note the mount point inside the VM, xvda1. There is no MBR/BIOS boot. This makes LV maintenance easier, no messing with kpartx. In theory, the LV could be exported and imported into a physical machine's VG.
4/. Line 47 uses app-benchmarks/pipebench. If it isn't available, then a simpler method of tell-the-user-something-is-happening is used.
5/. Not visible in the script, but I use nfsmounts for /usr/portage (and thus binpkgs), and fstab will make use of /swapfile if it exists (off by default).