IV stands for "idol video". Much like Japanese porn stars have "AVs" (adult videos), gravure idols have an abbreviation for their videos as well. However, while Japanese porn stars are referred to as "AV stars" or "AV idols", a gravure idol that puts out an IV will never be referred to as an "IV star" or "IV idol".
An IV generally features one gravure idol with a camera following her around while she poses or plays around on a beach. They will occasionally have interviews and almost always subpar music.