k9w8 - full lightweight IOS (this is what is bundled in the WLC .aes image, and is factory installed on “mesh” APs) rcvk9w8 - lightweight recovery image – this is factory installed on lightweight APs, unless a “mesh” image is specified; it lacks radio firmware.
RCV stands for RECOVERY. It's like the "EX" IOS in newer Cisco Catalyst. This software is an interim. The main function of this particular version is to ensure the AP can find the WLC through a series of search mechanism. No way is the RCV version meant for production.
Once the AP is able to find the correct WLC, the WLC will them push the "full" or production IOS version, the "k9w8".