Hello people !
I just installed successfully Ubuntu 10.04 under Xen with the help of David Markey and his pygrub 4.0.1! (Thanks again)
But I have a problem when I connect on this VM with VNC...
The mouse pointer is frozen on the middle of the screen. The click is ok but not the movements.
I tried these options : 
name       = "ubuntu"
bootloader = "/opt/pygrub64/bin/pygrub"
memory     = 512
disk       = [ 'phy:/dev/VolGroupHiro/VM1,xvda,w' ]
vif        = [ 'bridge=xenbr0' ]
vcpus      = 1
vnc        = 1
vncconsole = 1
sdl        = 1 
usb        = 1
usbdevice  = 'mouse' or 'tablet' or 'host:046d:c00c' 
For the last, I found it in /proc/bus/usb/devices
Someone has an idea to help me ? 
Thanks a lot ;) !