Since vyatta doesnt offer a download link to a PV image anymore, we need to convert the machine from HVM to PV manually, here are the step in order to achieve that:
1. Install vyatta from the vyatta-livecd-virt_VC6.3-2011.07.21_i386.iso image, image or system based images work, make sure to prepare the system for PV conversion at the end of the installation
2. Reboot and mount the xenserver iso from the CLI:
sudo mount /dev/disk/by-label/XenServer\\x20Tools /mnt
sudo /mnt/Linux/install.sh
3. Reboot and go to the Xenserver host console
xe vm-list power-state=halted (get the uuid for vyatta VM)
xe vm-param-set uuid=<Vyatta UUID> HVM-boot-policy=
xe vm-param-set uuid=<Vyatta UUID> PV-bootloader=pygrub
xe vm-disk-list uuid=<Vyatta UUID> (copy VBD disk UUID)
xe vbd-param-set uuid=<VBD UUID> bootable=true
Enjoy!