Guests which are not running (and aren't paused) do not show up neither with xm list nor in the Virtual Machine Manager. You have to "create" them again with xm create guestname. Redhat is working on an extension to the Virtual Machine Manager.
I was not able to install Windows XP with Xen 3.0.3 (Fedora Core 6) successfully. The basic install went smooth but after the first reboot, only a blank screen was displayed. AFAIK there is an issue with the ACPI implementation for HVM guests in XEN which causes Windows to choose the wrong HAL ("hardware abstraction layer").
The solution is to choose "Standard PC" in the beginning of the Windows install process so that a generic HAL is installed. This can be done be pressing F5 when Windows ask to press F6 to install RAID drivers. Unfortunately, my keyboard didn't work at this time of the install process... A workaround was to use qemu from Fedora Extras (yum install qemu) and do installation in the emulator with qemu -cdrom /home/fs/xp.iso -m 512 -boot d -hda /home/fs/vms/winxp. After that I used this image with Xen.
To boot a guest from CD, you need at least a cdrom entry in your guest configuration file (/etc/xen/...).
disk = [ 'file:/home/fs/vms/fc6_x32_build,hda,w',
'file:/home/fs/knoppix.iso,hdc:cdrom,r',
]
# c = hard disk, d = cdrom
boot="dc"
Paravirtualized guests can not be booted from CD.