URLs and URNs are subsets of URIs.
There are many answers on Stack Overflow that explain the concepts quite nicely. You often find a Venn diagram where URNs and URLs are overlapping sets, implying that each of the three areas are not empty. I can imagine a URN that's not a URL: the ISBN of a book. I can also imagine a URN that's also a URL: a normal web address.
What would be examples of URLs that are not URNs?