Codes are systems of signs that create meaning.
Conventions are the generally accepted ways of creating media, and are familiar to the audience. Conventions can be specific to a whole medium or to a specific genre.
Genres are categories of media (or literature or music or art) characterized by similarities in form, style, or subject matter. Some genres exist across many mediums, such as horror and mystery. Other genres are more specific to one medium, for example, sitcoms and procedural dramas are TV genres.