Grindcore, Fastcore, and Powerviolence: What Is It?

I can't count the number of time's I've been asked; "what's grindcore?" or, "what makes this powerviolence?" or, "what the hell is fastcore, how is it different than powerviolence?". And usually, I feel like I can give a pretty legitimate definition about these genres, but that being said, it has kind of gotten to a point where even seasoned listeners can get a little tripped up pinning down what subgenre a band falls into. It seems every band with a blast beat these days is labeled as a "grindcore" band, and every fast band with a sludgy part is "powerviolence". So, let's put it all to rest, I present to you, my take, and not very encyclopedic definitions of these three genres. Hopefully we can reach some level of understanding.