Ultraviolet is a form of light radiation that is harmful to our skin (think microwaves but from the sun, I wouldn't want to be cooked by that, you know? - this is a rough analogy), meanwhile ultrasound is basically just energy taken form in vibrations that are out of the range of the detection of our ear hair follicles within the inner ear (we don't get hurt by sound unless the energy of it is so excessively loud that it ruptures our ear drum, 140-160 dB).
Another more technical way could be the differences in their wavelengths. Sound has drastically longer wavelengths than light, which means that it has a harder time penetrating solids like ourselves since it doesn't have as much energy... we act as a dampener. Meanwhile ultraviolet light has so much energy that it forces our skin to absorb some and is at just the right wavelength to cause chemical reactions.