Son of former Manchester City and Portsmouth forward Lee, the current manager of the Saints` league rivals Havant & Waterlooville, the towering attacker spent three years at Fratton Park before joining the ranks at Watford after a successful trial period last season.
Nineteen-year-old Harvey played for the Hornets in a behind-closed-doors friendly during pre-season and immediately caught the eye of Saints manager Ian Allinson.
Allinson told his club`s official website: “Harvey is someone we’ve been keeping an eye on and he currently brings a fresh approach to the team in terms of his height.
“I’ve gone on record as saying that I won’t be bringing anyone in to the side for the sake of it and in Harvey’s case he is certainly someone who will improve the side and offer us something different in the final third.
“I have to thank Watford for agreeing to let us secure the deal until the end of the season as I’m sure it will prove fruitful for all parties”.