In the strictest definition it is only Half-Duplex since send and receive are on the same RF channel, but from the users point of view it is FULL-DUPLEX, since ZigBee and IEEE802.15.4 use a CSMA-CA (Carrier sense multiple access with collision avoidance) channel access mechanism.
Even in fairly busy networks, data can be sent a received almost instantaneously.
Regards,
- Ware