I don't think I've heard Power Metal described as an extreme metal sub-genre. You might be thinking of Doom.
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I've heard every Metal subgenre but Hair and Traditional as Extreme.
I'm no expert, but I've always been under the impression the main sub-genres of extreme metal are Thrash, Death, and Black, with Power and Doom being debatable.
Opinon:
There are two many subgeneres of genres, that in their own right, are subgenres.