Hi there
Xbee can be interfaced with 8051 using its USART section i.e. Tx/Rx pins. Code remains similar as for other USART devices such as bluetooth or RF, etc.
Just connect Xbee's Tx and Rx with 8051's Rx and Tx respectively. Also make sure to communicate on same baud rate. I will also suggest you to have a look at DIGI'd X-CTU software which is specially made for reading Xbees and its related internal information, you will find this software really helpful.
Few links that might help you as well:
1. "engineersgarage.com/microcontroller/8051projects/interface-serialport-RS232-AT89C51-circuit"
2. "engineersgarage.com/usart/tags/usart"