I have seen this kind of topic more than once in my many years of playing varying forms of MMO's and PvP type games, and there is a difference between PvP and Griefing, but the very concept of PvP is considered to be Griefing to some. So with that said.
PvP is by its nature a play style of opportunity, striking when the "Irons Hot" so to speak which looks to maximize gain verse what will be lost in the venture. For the most part in PvP when one player attacks another there should be some form of resistance even if that resistance is to run away, and yes being attacked while building or mining sucks, but that is the nature of this play style.
Griefing comes in many forms and stems from PvP in that it is also an opportunistic play style, but the major difference is that there is no intent for true PvP just one player camping, harassing, or just plain annoying another player, or worse yet trying to get the attacked player to leave the server for reasons not specified (We know this happens). It is one thing to declare war on someone and openly fight them, and conquered them wiping their faction out, it is even more epic for them to fight to the bitter end, maybe even causing a truce or alliance to be formed. For the most part with Griefing there is very little intent to gain resources, Alliances, or broker any form of player interplay and more of an intent to annoy or harass. I should know as I have been on both sides of Griefing in MMORPG's, (A vicious Cycle if you ask me), when I chose to grief it was with the express intent to ruin someone's day, and usually after I had someone camp me to the point I logged off. Then I would log back in and the first poor soul I found would suffer for the indiscretions of my harasser. Thus I was no better than the person who had made my life miserable. Simply put Griefing is a form of PvP, but it is PvP with the intent of annoying, harassing, or just plain ruining someone's day.
The Pirate/Raider play style is also an Opportunistic play style, with the intent to gain resources through intimidation, raiding, or protection (Mob style Criminal Empires. ect). The concept here is to have someone else do the work while the Pirate/raider turns a profit with as little effort as possible. Thus the Pirate/Raider has no desire to wipe out a faction that it can benefit from, and most of the time will not hassle a faction that is not seen as holding any value. (Though I would make this distinction)