The 18-year-old has yet to make a first-team appearance for the Bluebirds but finished last season as top scorer for the Under-18 side.
Speaking to the club's official website, manager Dave Jones said: "The loan will give Nat some good experience, will help him make progress and I'm sure he will do well at Southend."
"He is a player I have seen a couple of times," Sturrock told the club's official website. "His attitude to play is excellent and he is very quick so that will benefit the team."
Jarvis signed the loan deal after a successful trial match for Southend reserves in midweek and is in their squad to face Hereford on Saturday.