How to recover VM on Xenserver When VM is not bootable from updated kernel (Citrics XenServer )

1 . Create/Restore an new VM (say rescue vm )on the xenserver where the crashed VM is located.

2 . Stop the  rescue  vm and deattach the storage device .

3 . Attach this storage  device on crashed vm .

4 . Change the crashed disk vbd to non bootable after checking with below command

    xe vm-disk-list vm="crashedvm name" l ( It will list the storage devices with their uuid's ,for each disk there is a vdi and vbd uuid )

  xe vbd-param-set uuid=7f6a1563-fc76-31d8-49c2-c03466ca836b bootable=false

5 . Change the rescue vm disk vbd  to bootable .

  xe vbd-param-set uuid=5f64a143633-fc76-313-59a2-d13466ca3egf bootable=true

6  Start the crashed vm ,it will boot with the rescue vm's disk.
    Now ,mount the  original disk  whose vbd you  changed to non bootable earlier .

    mount /dev/xvda /mnt

7 . Going  to /mnt ,we will get the old filesystem.
     cd /mnt/boot/

    Changed the grub.conf default  entry   to 1 i.e the older version of kernel is selected( as i have updated the
    kernel after which vm  does not starts)

8. Stop the this crash vm again .

9 . Deattach  the rescue disk from the crashed vm and change the crashed disk's vbd again to bootable .
     
       xe vbd-param-set uuid=7f6a1563-fc76-31d8-49c2-c03466ca836b bootable=true.

10 .Now try to start it  and now it is  bootable (from earlier  kernel.)