vif         = [ 'model = e1000' ]
vfb         = [ 'vnc = 1,vnclisten   = 0.0.0.0' ]
sdl         = 0
vnc         = 1
serial      = 'pty' 
usbdevice   = 'tablet'
bootloader      = '/usr/bin/pygrub'
kernel          = '/usr/lib/xen/boot/hvmloader'
builder         = 'hvm'
boot            = 'dc'
disk            = [ '<target disk spec here>,xvda:sda,w', 'file:<path to your ISO here>,xvdc:cdrom,r' ]
After you start the VM, open up a vncviewer to the dom0 address and you should see the install screen.