If you cancel a friend request coming to you, then the person sending the request will know about it if they check. They can check by visiting your profile and then see what is the status of their request. If it is still as Friend Request Sent, then it means the request is pending, not cancelled. If it again shows as Add Friend, then they can know that you have cancelled their request. However, they will not get any notification right at the moment when you cancel the request.
If you cancel a friend request sent by yourself to someone, the person receiving the request will know about this only if he is online between the time you sent the request and you cancelled the request. They will receive a notification of a friend request and if you cancel it before they respond, they will know as they will not be able to accept the request.