The other day I needed a small OS to install on KVM just to test that a VM got the right DNS name. I noticed there was a small Linux Distribution called Cirros available in virt-builder. There were a few small issues starting the image though. During bootup it tries to load metadata for the VM from the cloud provider it is running on. This makes the bootup quite slow.
Linux KVM provides a pretty neat virtualization solution on linux. It performs pretty well, and there is a quite a collection of tools like virt-builder, virt-install and libguestfs to help one automate all kinds of tasks. If you have a bunch of virtual machines on KVM you often end up with or accessing them by IP or by maintaining a list of entries in /etc/hosts. Both of them are not ideal.