Sir I haven't understood the answer to the Q (How to acheive this full duplex comunication of xbee with 8051 uc ?

I don’t know what your question is. But simply if you remove the radio out of the equation and connect wires between DI and DO of the two devices, you should be able to send and receive data. Once you have your application working in this fashion, then and only then are you ready to “Cut the wires” and put the radios into the system.