Not all hardcover books have sewn bindings. And not all paperbacks have glued bindings.
Since about 1970, more and more hardcover books had been made with glued bindings. The glues are better than those used in the old paperbacks - they will remain flexible for quite a while - much longer than the paperbacks. The reason for doing this is that it costs less to produce a book with a glued binding than a sewn one. With lower production costs, it allows a publisher to offer a hardcover book (which is more desirable) at a lower cost.
In perhaps the past 10 or 15 years, paperbacks with sewn bindings have become more available. These books combine the advantages of a sewn hardcover book with the size and weight of a paperback. They are more expensive than normal paperbacks, but cheaper than sewn hardcover books. This sort of binding is generally used for books that will receive a lot of use and where space is an issue, like guidebooks.