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What it does: viscosity controlling, emulsion stabilising

Also called: poly vinyl pyrollidone, poly N-vinyl-2-pyrrolidone 

The science

PVPs stand for Poly Vinyl Pyrollidone, a big molecule created from repeated units of Vinyl Pyrrolidone, aka VP. Its main purpose is being an important film former. It was the first synthetic polymer introduced as a hair fixative in the 1950s. It is an excellent film-former that is substantive to hair, forms clear films, and is completely water soluble. 

PVP is used as a fixative to hold hair together. This is what keeps curls in place and defines curl pattern. It will most commonly be found in gels, mousses, hairsprays, and holding products. 

PVP attaches itself to surfaces such as the hair and the skin and forms a nice, thin, even film there. The film is useful for holding a hairstyle or extending the wear of color cosmetics and sunscreens. The disadvantage of PVP is that the film is a bit brittle and that PVP loves water (hygroscopic) that tends to destroy the film. This is the reason why hair styled with a PVP based product may lose its style in high humidity. 

Benefits for your hair

Keeps curl pattern intact 

When a product with PVP is applied after using moisturizing and styling products, the PVP will form a thin film around the hair strands, holding the curls in place and keeping the definition. 

TLDR 

PVP is an important component of curl care. It is the agent that helps keep your curls intact and gives them that lovely definition. Without PVP, you may be washing and styling your hair a lot more often than necessary!