Humans have rods (light sensors) and cones (color sensors) in their retinas that allow us to interpret received light in terms of intensity and color. Presumably, there are structure in Kryptonian retinas that are sensitive to x-rays. They're right next to the ones that don't receive light, but generate infrared light that is focused by the lens to create heat vision.

Just like rods and cones in the human eye, Superman possibly has x-ray detecting crystals like Silicon or Cadmium-Telluride in his eye that detect x-rays passing through a special lens called Kumakhov polycapillary focusing x-ray lens implanted in his eye.

The other possibility could be that x-rays get converted to normal light by a film of x-ray fluorescent material and then it is the normal work of the rods and cones like in case of the human eye.

Superman's eyes actually PROJECT X-rays; depending on how much is absorbed or reflected back at him allows him to see through solif objects.

Back in the day, Superman's "heat vision" was actually just a creative use of his X-ray vision -- he would project enough X-Rays to actually melt or destroy an object.

magic

but does he always see in xray?
and if he doesn't how does he switch between xray and our normal sight?

In today's society, he probably couldn't use it with all the lazy people with lead in their asses! LOL

you are a all a bunch of freaks superman does not exist his x ray vision works becuase the writer made it that way GET A LIFE!!!!!!!!!!!!!!!!!!