Here's a Xen command line cheat sheet. We'll assume Xen is installed.
1. Make sure you've booted from the Xen kernel.
2. To start the Xen daemon, use "xend restart".
3. To install a new image, use virt-install --paravirt --name vc8 -r 500 -f /var/lib/xen/images/vc5.img --file-size 8 -l http://fedora.fastsoft.net/pub/linux/fedora/linux/releases/8/Fedora/i386/os --nographics
The virt-install man page has more examples and an explanation of the above. The above command installs Fedora 8 on the image. You should be prompted to install the OS in the VM after the download. This can obviously take a while, so you'll want to clone these images.
4. To list vms, you can use "virsh list" or "xm list".
5. To start your VM, use "virsh start vc8". The image "vc8" was the image created above.
6. To connect to the above VM, use "xm console vc8". Replace vc8 with your VM name. This will boot the operating system, which should get you to a login prompt.