"Careful what you wish for," the president said. The prince responded, "You can dish it out, but can you take it?"
Legend says the 105-carat Kohinoor diamond should only be worn by a woman or a god.
The late comedian's fans, including David Walliams, had long campaigned for him to be knighted.
The occasion will be marked with a healthy dose of British understatement.
Adams is leader of the former political wing of IRA, which killed the prince's great uncle 36 years ago.
Political neutrality of future king at risk as decade-long wrangle with the Guardian newspaper over 27 letters resolved.
Cameron seemed unaware that microphones were picking up his conversation with former New York mayor Michael Bloomberg