Both China and North Korea confirmed that Kim Jung Un traveled to Beijing earlier this week for meetings with Xi Jinping. The White House even released a statement that Xi had sent a message to President Trump about the meeting.
This was Kim's first trip outside of North Korea since taking power in 2011. I guess it would have been a serious breach of diplomatic protocol if Kim were to travel across the Military Demarcation Line to meet South Korean President Moon Jae In or meet with President Trump without first meeting with Xi. After all, the Chinese Communist Party leader is Kim's chief sponsor and political boss.
Does this mean North Korea is serious about meeting the U.S.? The statement on that issue is preceded by a very long "if."
"If South Korea and the United States respond with good will to our efforts and create an atmosphere of peace and stability, and take phased, synchronized measures to achieve peace, ..."
So it's up to South Korea and the U.S. to see that things go forward from here. North Korea maintains the option to throw a wrench in the process at any time.