'Courtesy" is the name given to the customs and behaviours of expressing politeness. 'Courteous' is a description of a person who expresses the behaviour of politeness. SO, when describing a polite person, the person 'IS COURTEOUS' You can also say, 'THEY HAVE COURTESY'. I hope this helps.

To build on Edgar's answer: Courtesy is a noun when it means polite. (It has another meaning, somewhat like "provided as a benefit" which is an adjective": a "courtesy car." Courteous is always an adjective.