Thanks @Alec_t ,I changed the V1 voltage to about 2.5v , Output now is 26v (though I have designed the system for 20v) and it varies drastically with change in the input from 12 to 15v.
Its actually due to resistor R1 being too big, I changed its value to 10k and changed all other compensation elements for specific Phase Margin and it started giving correct output.
I am highly thankful to all your helpful advises and thoughtful suggestions.