It depends on the Pokemon your using.
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Hidden Power is a move that varies in type depending on what pokemon has learned the move it is random on what type it will be in can vary from 16 different types in most pokemon games this is a Technical Machine you can recieve then use in others it's a move tutor move.
If your Pokemon can't learn grass, it's your only hope, but Unown only has one move which is Psychic Hidden Power, so I think Psychic's hidden power is better than Grass.
unown's hidden power is simple: it works the same as any other hidden power. so if your in a cave i think the move's type is dark, on or under water is water, forest and grassland is grass, on rocky ground of course is ground or rock. I've played since 2003 and I am still not sure.
Depending on the pokemon, it will randomly pick a type and strength of the move. It is a very unique attack.
In Lilycove City, there's a move deleter (near the department store) that can delete any move from a Pokemon's repertoire, including HM's, Hidden Machines.