Hu's visit comes amid US pressure for Beijing to do more to get its communist ally North Korea to stop developing nuclear weapons.
"Hu Jintao's visit is in part to encourage a breakthrough at this very important moment," said Shi Yinhong of the People's University of China. "China wants North Korea to show more flexibility."
The official Xinhua News Agency said Hu arrived in Pyongyang, the North Korean capital, at midday Friday, but gave few other details.
It described Hu's trip as an "official goodwill visit to the Democratic People's Republic of Korea," using the North's official name.
Hu was accompanied by China's Foreign Minister Li Zhaoxing and other officials, Xinhua said.
Li spoke by phone on Thursday night with US Secretary of State Condoleezza Rice, the Foreign Ministry said. It said they discussed international affairs but did not give details.
The visit is the first to North Korea by a top Chinese leader since 2001 and is due to last through Sunday.
New round of talks
It comes as China is trying to organise a fifth round of six-nation talks scheduled for 8 November on demands that North Korea give up nuclear development.
Other nations taking part in the talks include the US, South Korea, Japan and Russia.
Talks held last month in Beijing ended with a promise by Pyongyang to give up its nuclear programmes in exchange for aid and a security guarantee.
The six-party talks also include
US, S Korea, Japan and Russia
The challenge now is to begin implementing a landmark joint statement the parties agreed at the last session.
North Korea agreed in the blueprint document to dismantle its nuclear weapons programmes and rejoin the Non-Proliferation Treaty in exchange for aid and better ties with Washington and Tokyo, but questions remain over the timing of concessions.
There are also disagreements between the six parties over Pyongyang's demands for a light-water reactor to generate atomic energy.
Striking a tough note, a North Korean diplomat told the South's Yonhap news agency on Thursday that the Communist state would not give details about its nuclear programmes and atomic weapons until the light-water reactor had been built.
Pyongyang wants aid in return
for ending its nuclear programme
Han Song-ryol, deputy chief of the North's mission to the United Nations, also said Pyongyang was not interested in Seoul's offer of energy supplies if it was meant as an alternative to the reactor.
The nuclear crisis erupted in late 2002 after US officials said North Korea admitted violating a 1994 deal by embarking on a secret uranium enrichment programme.
Some observers saw Hu's three-day visit as less about details than as a gesture of friendship between long-standing allies.
"For the most part it is symbolic, to show China's policy towards Pyongyang is consistent and stable and it attaches equal importance to relations with both the North and the South," said Niu Jun, a professor of international relations at Peking University.
"And Pyongyang certainly needs the international friendliness and goodwill."
Analysts said there could also be items on the agenda apart from the nuclear issue for the Communist neighbours.
China is the North's biggest provider of food and fuel aid, a role that could become even more crucial in light of an announcement last month that North Korea no longer wanted to rely on international organisations for humanitarian aid.
"Pyongyang certainly needs the international friendliness and goodwill"
Professor of International Relations at Peking University
With speculation mounting over who might succeed Kim as leader, analysts said the trip could also be an opportunity for North Korea to introduce Hu to the chosen successor, who will almost certainly be one of Kim's sons.
Kim, who rarely ventures out of his country, last visited Beijing in April 2004 amid tight secrecy.