Qcow2

Create a disk image or download one from Stacklet.com

dd if=/dev/zero of=/srv/xen/CentOS5.img bs=1024 count=10000000

qemu-img convert -O qcow2 CentOS5-base.img CentOS5-base.qcow2.img

qcreate-qcow 50GB CentOS5-base.qcow2.img CentOS5.qcow2.img