The 1.93m, 91kg Neath-born flyer qualifies to play for Ireland as his grandmother hails from Cork. He played for the Ospreys against Connacht in the Magners League last November.
Speaking about his new deal, Jarvis said: "I am very excited at moving to Ireland and playing for Connacht. Connacht have a long history of developing exciting young talent who have gone forward to play for the national team.
"I am also looking forward to working with and learning from Eric Elwood. He was a great 10 for both Connacht and Ireland and if I can't learn and develop from Eric then I'm in the wrong job.
"My grandmother was born in Cork. I have always considered myself as 50% Irish, this has now presented me with an exciting opportunity to play in Ireland and hopefully one day for Ireland."
James Loxton will also join Connacht for next season. The 1.75m, 84kg utility back can play on the wing or at full-back. Loxton, who was born in Berlin, qualifies to play for Ireland as his mother is from Crossmolina, County Mayo.
The 21-year-old has been capped by Wales at Under-18 and Under-20 level, and lined out against the Ireland U-20s last year in Athlone. He has been signed from the Cardiff Blues.
"Connacht have big plans for the future and I want to be part of that vision. They play an open, expansive and exciting brand of rugby which will suit my game as a wing/full-back.
"I would like to thank the Blues for developing me from a junior International to a regional player and I look forward to continuing that progress in Ireland.
"My mother was born in Crossmolina in Mayo, I go on holiday there every summer and I'm delighted at the prospect of playing professional rugby where my ancestors were born and bred."
Commenting on the province's latest acquisitions, Connacht Rugby CEO Gerry Kelly said: "We are delighted to announce the signing of these two very promising young players.
"They have both shown they are ready to make the next step in their careers having featured for Wales at youth level and their respective clubs.
"We have always said that we are willing to give players an opportunity to show their skills here at Connacht and we hope that they can go on and follow in the footsteps of long-serving Exiles such as Adrian Flavin, Mike McCarthy and Michael Swift.
"They are two very exciting young players and we are confident they will fit comfortably into our squad. We are also very happy to be bringing Irish-qualified players into Connacht and hopefully they can progress to the national set-up in the future."