Most of materials that highly reflect the visible spectrum of light are highly absorbing materials for UV- spectrum, like noble metals(Au, Ag and Cu), and also alkali metals. To have a mirror that reflect light in a long spectrum we can use multilayer mirrors that have constructed from complex of several different layers with thickness of several nanometers,(coated on a substrate) that known as VUV-UV Mirrors .
You can search for “multilayer uv mirrors:pdf” in net to find something like this:
Design of multilayer extreme-ultraviolet mirrors for enhanced reflectivity
Whatever the range of spectrum required for reflection increases, the number of layers increase.