KDE is a desktop environment, not an entire operating system. KDE is somewhat like what Windows was back in the days when Windows ran on top of DOS. DOS was the Disk Operating System, and Windows was an environment that ran atop it.

Debian is a Linux distribution. Meaning that it is Linux--a small piece of an operating system, not useful by itself--plus a selection of other odds and ends that fit together to provide an operating system and a wide selection of applications that run on that operating system.

KDE can be installed and run on Debian.