Chivers said he had wanted to return to the ring with a tough fight and Belsham was a game opponent.
'He took some of the best body work I've ever put together in a fight.'
The judges all had Chivers ahead on points after the five-round contest.
It's his second world title.
Chivers beat five opponents in a six-day tournament to win the middleweight world amateur IMMAF title in Las Vegas at the end of 2016. That was part of UFC International Fight Week and he also competed in the International Brazilian Jiu-Jitsu Federation World Championships that year.
He won his first professional MMA fight in the Shuriken series in May 2017, defeating Muaina Manase.
He had a frustrating two years after that but has emerged from his struggles in good shape.
'I feel good. I've put on size and I'm physically fit and strong.'
Chivers will compete in freestyle wrestling. Along with Greco-Roman, it is one of the two styles of wrestling contested in the Olympic Games.
He learned his craft at Te Kura Awhio in Gisborne and has been training in Auckland under Steve Oliver, long-time coach of Kiwi MMA fighter Mark Hunt.
Oliver said he was not surprised by the kickboxing result.
'I was very confident Byron was in the right place, mentally.'
Chivers had now built a strong base in martial arts and his specialist wrestling training would kick into high gear.
'He's stronger than he's ever been,' Oliver said.
'His fitness is at elite standard.'