The Arduino board receives serial orders from a Raspberry to drive motors. I would like the Arduino to simultaneously receive some XBee signal. Is it possible?
Yes, it is possible. How depends on which Arduino that you want to use.
I suppose it is limited to Mega and Due.
The Mega has multiple hardware serial ports so the baud rates can be high on both the serial devices and the Mega can receive on multiple ports at the same time. You can have 1 port for Pi, one for the Xbee and another for program upload and debug output. The Due, I don't know enough about to comment. I would rule out an Uno (or any Mega328 based board) because of the requirement of simultaneous reception from two devices and the low baud rate allowed by software serial. Only one software serial port can listen at a time.
Leonardo and Micro have an serial port that's independent of the USB.