There are several open-source third-party tools available for creating new virtual machine images.
Oz is a command-line tool that has the ability to create images for common Linux distributions. Rackspace Cloud Builders uses Oz to create virtual machines, see rackerjoe/oz-image-build on Github for their Oz templates. For an example from the Fedora Project wiki, see Building an image with Oz.
VMBuilder can be used to create virtual machine images for different hypervisors.
The Ubuntu 12.04 server guide has documentation on how to use VMBuilder.
VeeWee is often used to build Vagrant boxes, but it can also be used to build KVM images.
See the doc/definition.md and doc/template.md VeeWee documentation files for more details.
