I've repeatedly noticed that many people have a very difficult time with a particular realm of metal. Viking/pagan/black/folk categorization fiascoes abound, and debates spring up endlessly over where the lines are, which side of the lines a particular band is on, and if the lines even exist. Well I have taken it upon myself to create a little Venn diagram which will clear everything up for you once and for all. If you find yourself asking which of those four classifications a given band falls under, and you just can't seem to decide for sure, then simply refer to the drawing and your question shall be answered.
You'll notice I also threw in most of the other main sub-genres of metal, but that was mostly a "why not?" since I was already playing on MS Paint. And you'll also probably notice the lack of some connections, like death-doom. I suppose I could add a little rainbow bridge or something, but I'm too lazy for that.
Anyway, this should solve your problem.