The men were snatched by a group of unidentified gunmen as they fished in waters running between China and North Korea on May 8, and held for 13 days, according to earlier reports and witness accounts given to AFP.
It remains unclear who was behind the rare abductions. Xinhua said that no ransom had been paid and the men were released after "diplomatic exchanges" between Pyongyang and Beijing, the North's only major ally.
The state-run Global Times daily urged Pyongyang to clear up the question of who was responsible for the kidnapping, saying China would not tolerate "any misbehaviour by North Korea".
It quoted residents of the north eastern port city of Dalian, where the victims were from, as saying North Korean coastguards had in the past captured fishing boats and stolen fuel and other items on board.
"North Korea should respect China's every concrete interest, especially the lives and property of Chinese citizens," the paper said in an editorial.
"More details should be disclosed regarding who was really behind the abduction and the conditions of release," it added.
The China Daily newspaper also said no ransom was paid for the men's release, although earlier reports said their captors had demanded money in exchange for their return.
China is North Korea's key source of economic support, but Beijing has appeared at a loss as to how to rein in recent provocative behaviour by the isolated state.
The incident came after Beijing criticised a recent North Korean rocket launch and expressed concern over a nuclear weapons test reportedly being readied by the isolated nation.
Chinese fishermen regularly run into difficulties with the authorities of other countries as they fish in areas that are claimed by both China and its neighbours.
Chinese state media previously reported the snatched men were fishing in Chinese waters when they were seized.