If you want to play around with Nova in the cloud<<FootNote("Yo dawg, I heard you like clouds, so I put a cloud in your cloud..")>>, you can use user-mode-linux instead of KVM or Xen. Here's how:
First of all, you need an image that can be run by UML. Grab an Ubuntu Lucid one here: http://nova.openstack.org/~soren/ubuntu-lucid-uml.img.gz
Bundle it up like you usually would:
wget http://nova.openstack.org/~soren/ubuntu-lucid-uml.img.gz gzip -d ubuntu-lucid-uml.img.gz euca-bundle-image -i ubuntu-lucid-uml.img euca-upload-bundle -b uml-image-bucket -m /tmp/ubuntu-lucid-uml.img.manifest.xml euca-register uml-image-bucket/ubuntu-lucid-uml.img.manifest.xml
This gives you back an AMI id. Nova doesn't (yet) understand that this is a UML image and thus doesn't need a separate kernel, so when you want to run it, you need to pass a kernel and ramdisk to euca-run-instances.
Explaining to nova-compute that you want UML
--libvirt_type=uml in /etc/nova/nova-compute.conf