They are interchangeable. They can both be nouns and adjectives. I have psychic powers. I am a psychic. I have clairvoyant powers. I'm am a clairvoyant. http://www.merriam-webster.com/dictionary/clairvoyant http://www.merriam-webster.com/dictionary/psychic

'Psychic' is an adjective meaning having the ability to see things that are usually beyond normal senses. It can also be a noun. 'Clairvoyant' is a noun meaning a psychic person. You would seldom talk about a 'clairvoyant person' - it sounds redundant, even if it's correct. They are very close, then, but have slightly different usage. Most of the time they are interchangeable.

Correction! Psychic can be a noun. Sorry about that. I goofed.