Hardcore
Hardcore larping can imply any kind of extravagance undertaken in the name of larp (e.g. "he's such a hardcore larper he knits his own woolen underwear"), but more often describes a particular approach to larping where the player avoids simulation also when dealing with emotionally difficult or physically harmfull actions.
Hunger is portrayed by being hungry, fighting by actually fighting, being hurt by being hurt, and falling in love as a character by falling in love as a player. The bleed concept is meaningless when applied to hardcore larping, as the player consciously tries to dissolve all distinction between player and character.
Limitations and framework
The kutt and brems rules are usually the only limits to how far a player may go when playing hardcore, though common sense obviously applies and players often spend a long time talking through their personal limits and establishing trust before entering a hardcore larp.
As hardcore is defined by players simulating less than what is considered normal in a particular larp tradition, the threshold of what is considered hardcore will vary from group to group and time to time.
Larp organizers may declare that a particular larp is meant to be played in a hardcore style, though it is more often an option availble to those players who choose it, or an approach chosen by players irrespective of the wishes of organizers and co-players.
Player motivation
Those who pursue hardcore play usually give one or more of three reasons for their choice: intensity of experience, the trust and emotional bonds formed between players who go through such an experience, and a notion that it is disrespectful to distance oneself from certain themes through simulation..
Controversy
There is, indeed, controversy surrounding hardcore larp. For example, the name "hardcore" is poorly chosen.