According to the Korean Central News Agency, Kim was spotted traveling with his wife and various high-ranking officials including Kim Yong Chol, who is the vice chairman of the Central Committee of the North Korean ruling Workers' Party.
Last year, Kim travelled to China three times to speak with Mr Xi before and after summits with Trump and South Korean President Moon Jae-in.
Christopher Hill, a former USA ambassador to South Korea, said Kim's visit to China may be Beijing's way of ensuring it remains a player in any future developments with Washington.
South Korea-based analysts and North Korean defectors report that with sanctions still blocking most coal exports, the North has put more of its stockpile to use domestically.
In the Chinese border city of Dandong, dozens of security vehicles and officials blocked the roads around the station before the train passed through, Yonhap reported, before reopening them afterwards.
The confirmation of Kim's trip to China came after South Korean media earlier reported that a special train carrying a high-ranking North Korean official crossed into China on Monday night.
Trump Reportedly Trying to End Shutdown Without Looking Weak
That effort is created to squeeze Senate Republicans, some of whom are growing increasingly anxious about the extended shutdown. Speaking on CNN's "State of the Union", Intelligence Committee Chairman Adam Schiff called the idea a "nonstarter".
Trump and Kim met to discuss denuclearizing the Korean peninsula last June in Singapore, but little headway has occurred since then.
Earlier this week, Mr Trump said a location for another meeting between the two would be announced in the not-too-distant-future.
"Kim's confidence stems from the expectation of growing and reliable support by China", Rudiger Frank, a professor of East Asian economy and society at the University of Vienna, wrote on the North Korea-watching 38 North blog on January 2.
The latest letter from Kim came as denuclearization talks remain at an impasse and after the North Korean leader warned in a New Year's speech that Pyongyang may change its approach to the talks if Washington persists with sanctions.
About 30,000 North Koreans have fled repression and poverty and settled in the capitalist South, mostly by secretly crossing over the increasingly porous border with China.
North Korea's coal production rose from 2013 to 2016, but fell 30 percent to 21.66 million tonnes in 2017 from 2016, according to South Korean government data. Both parties signed a pledge at the time to denuclearise the Korean peninsular, though was never clear what this would entail.
The United States has been pressing for North Korea to get rid of its nuclear weapons before any easing of economic pressure.