RCV files are smaller than the fully functional k9w8 CAPWAP files. When booting rcvk9w8 files, the AP can join a controller to download a fully functional image. The AP will then reboot, use the fully functional image and rejoin a controller to obtain its configuration. if you are looking for details on lightweight registration to WLC then check here
what firmware you use , "RCV" firmware's main function is to boot up the AP so the AP can FIND a valid WLC.
Once the AP finds and joins a WLC, the first thing the WLC will do is push a full CAPWAP IOS to the AP. The AP will then use this new CAPWAP IOS (disregarding the RCV IOS).
Download the appropriate "K9W8" firmware from here then use the AP as LAP.