Recently I updated XenServer to 5.6 and run into some issues with Provisioning Services 5.1 SP2. Not all environments will encounter the problems and I'm unsure when exactly it happens.

Environment is XenServer 5.5 and PVS 5.1 SP2 where PVS runs also the PXE service. After updating one XenServer to 5.6 the VM's couldn't Network boot anymore from the 5.6 host. All other VM's on XenServer 5.5 still work fine. The error you get is "no filename or root path specified". With XenServer 5.6 comes a new PXE component (gPXE) in the VM BIOS and might be the problem. Workaround was to use the DHCP option 67 and not using the PXE service from the PVS.

After the DHCP change the VM boots from the vDisk but ends in a BSOD 7B Stop error. Using the Safe Mode boot showed a xen driver to be the cause of the BSOD but still the same vDISK still works fine for VM's on XenServer 5.5. Manually changing the driver and disabling the signing solved the BSOD. Now the right way to do it is to re-image the vDISK and update the XenTools and re-installing the PVS drivers (Network stack is key here).

So be careful!