Steroids.org reports that "Deca" refers to an anabolic steroid called nandrolone decanoate. The "300" refers to its concentration of 300 milligrams per milliliter, and the "QV" is the manufacturer's name.
According to Anabolic-Steroids.info, the initials "QV" refer to a company called "Quality Vet." The company was based in Mexico and supposedly produced anabolic steroids for the veterinary market, but it probably made much of its profit off the human black market for steroids. Counterfeit QV products were commonly offered on the black market, often packed in real QV packaging. The company Quality Vet went out of business in 2005 after a U.S. Drug Enforcement Administration operation.