Well, I noticed that one of my Server running Xen Server 6.0.2 does not have the option to boot up automatically on server boot. Later I came to know that Citrix has removed this option in newer versions.

I found a way around to fix this and enabled auto start using console.

First you need to create a pool on your server (if you do not have already) .

Code:
xe pool-param-set uuid=UUIDOFPOOL other-config:auto_poweron=true

Code:
xe vm-param-set uuid=UUIDOFVM other-config:auto_poweron=true

You can get the UUID of Pools and VM from general tab in xencenter.