How can i take snapshot of command prompt window in full screen mode
In full screen mode all you have is text. There is no graphical `rendering' as such. If you can capture the text, it is enough ... though you can always reconstruct a png image later from the text (if you really have to get an image out of it).
Why don't you just use an external screen shot software?
There's many, e.g. greenshot, which is free (is in speech and beer :-)).