From this thread:

Positive and Negative Energy: These two terms show up in a variety of abilities, but they have no definition outside those abilities, and the abilities aren’t always consistent. How do positive and negative energy work?

Positive and negative energy are two damage types, though despite their name, they are usually not included on the list of energy types you can choose with spells like resist energy or feats like Elemental Spell. You’ll sometimes come across both the phrasing “deals X damage; this is a negative energy effect” and the phrasing “deals X negative energy damage”; these two are functionally equivalent.

Positive energy often heals living creatures, though not always (for instance channeled positive energy to harm undead or the life blast spell). It often harms undead creatures, though not always (for instance channeled positive energy to heal living creatures). Individual effects will tell you whether they heal living (if they mention healing without specifying what they heal, they always mean only living creatures), harm undead, or both. Positive energy never heals or harms creatures or objects that are neither living nor undead (such as constructs), and it never directly damages the living or heals undead, barring some special effect that explicitly changes this like a dhampir’s negative energy affinity. These rules extend to the fast healing from positive-energy attuned planes as well (though overhealing on a major positive-energy attuned plane can be dangerous as well); only living creatures gain fast healing on such a plane.

Negative energy works just as described above for positive energy, reversing living creatures and undead in all cases (it often heals undead, it often harms living creatures, if it mentions damage without specifying what it damages, it always means only living creatures, and so on).