The upgrade can be done without cloning the machine but we need to make change the boot order from Citrix XEN host console.
Before starting upgrade.
xe vm-param-set HVM-boot-policy="BIOS order" uuid=[uuid of your vm]
-So that to get the BIOS option while booting.
Once Upgrade has started:-
Again we need to change the boot order to previous state , so that After first phase of installation It will boot from HDD .
xe vm-param-set HVM-boot-policy="" uuid=[uuid of your vm]
Note:-If the server contains more than two (/ and swap partition)
/boot swap and / partition or more than that , In second phase during upgrade It might not found xvda device which is device name for citrix xen VMs, and may look for devices hda and sda which cause the GRUB error and boot-failure.
I will write a separate cool solution for the same issue and will publish for the time being to proceed with the upgrades you can use the steps mentioned.