[3] Kim made his debut in 2003 as a member of TVXQ, a boy band produced and formed by South Korean record label and talent agency SM Entertainment, having previously been a trainee for six years.

Kim began his solo career in 2010 with the release of Japanese EP Xiah,[6] which peaked at number-two on Japan's Oricon singles chart.

After the release of Xiah, Kim's solo music career was sent into hiatus because of a discord between him and his Japanese label Avex Trax that led to the sudden suspension of all his activities in Japan.

They appeared on a show called "2002 Survival Audition – Heejun vs. Kangta, Battle of the Century", along with three future members of rock band TRAX (Jay Kim, No Minwoo, Kang Jungwoo).

The band comprises former label-mate female singer BoA, Tablo of South Korean hip-hop group Epik High and jazz pianist Jin Bora.

[20][21] On July 31, 2009, Kim and other two TVXQ members—Jaejoong and Yoochun—submitted an application to the Seoul Central District Court to determine the validity of their contract with SM Entertainment.

[4] Throughout his career as TVXQ's member, Kim had written and composed several songs that appeared on the group's Korean and Japanese releases.

His first composition was a ballad titled "Nae Gyeote Sumsuil Su Ittdamyeon (White Lie)" (네 곁에 숨쉴 수 있다면), included in TVXQ's third Korean studio album, "O"-Jung.Ban.Hap.

[24][25] At the beginning of 2010, Kim starred in the South Korean production of Austrian musical Mozart!,[7] playing the lead role Wolfgang.

[27][28] In April, 2010, the three-member group JYJ (initially known as Junsu/Jaejoong/Yoochun) was created, following an announcement made by Avex Trax, the trio's Japanese label at that time.

[5] After Avex revealed that TVXQ would go on an indefinite hiatus, it's decided that Kim would make his solo singing debut in Japan.

[6] A teaser showing his self-composed song "Intoxication" was made public and announced as the lead track of his upcoming single.

Kim promoted the new album with JYJ via a worldwide showcase tour with dates in South Korea, Southeast Asia and the United States.

Kim and actress Song Ji-hyo featured in the six minutes long music video for the album's lead track, "In Heaven".

To promote the album, he held concert tour kick off in Seoul on May 19 and 20 and continued to Thailand, Indonesia, Taiwan, China and Hong Kong.

The concert featured ballads from his album as well as OST songs and a few pieces from his previous musicals, accompanied by an orchestra during the whole show.

[47] By the end of 2012, Kim has sold almost 200,000 copies of Tarantallegra and Uncommitted combined, becoming one of the year's highest selling solo artists on South Korea's record chart Gaon.

In November 2012, the three years and four months lawsuit concluded as SM Entertainment and JYJ reached a mutual agreement to terminate all contracts between the two parties and not to interfere with each other's activities in the future.

At a voluntary arbitration at the Seoul Central District Court, SM Entertainment finally agreed that its contract ended July 31, 2009, and that it would no longer interfere with JYJ's endeavors in Korea.

[48][49] On February 25, 2013, Kim (as JYJ) was invited to perform at South Korea's current president Park Geun-hye's inauguration ceremony held at the National Assembly Building's Center Plaza, in front of more than 70,000 people.

The sold-out concert marks the first time JYJ had been able to perform in Japan after the contract with their former Japanese management agency Avex ended in 2010.

On the same day of the album release Kim launched 2015 Xia Third Asia Tour Concert – Flower[56][unreliable source?]

[62] Kim was discharged from military service on November 5, 2018, and started promotions with a fansigning event at COEX Live Plaza.

[65] Kim was announced to star in the musical Xcalibur as King Arthur from June 15 to August 4, 2019, at the Sejong Center for the Performing Arts, Seoul.

[79] In November 2022, Kim announced would held year-end concert titled 2022 XIA Ballad & Musical Concert with Orchestra Vol.8 on December 23 to 25 at COEX Hall D.[80] On July 6, 2023, Palm Tree Island announced that Kim would held a fanmeeting Cocotime in August embarked in Seoul, Yokohama, Osaka and Nagoya, to commemorate his 20th anniversary debut.

[81] For his 20th anniversary debut, Kim also released digital single "Red Diamond" on August 22, which contains Korean and Japan language.

The song was written by Broadway composer Frank Wildhorn, with lyrics by Tracy Miller and Carly Robyn Green.

[83] On June 12, 2024, Kim released digital single "Our Season", a "semi-rock ballad" that contains a message of support and consolation for imperfect youth.

[84] On September 6, 2024 at 12 AM KST, the two remaining JYJ members, Kim Jae-joong and Xia Jun-su, unveiled a teaser for their continued collaborative venture, JX[85][unreliable source?

The teaser features a voice-over narration by the duo, emphasising their longstanding support for each other amidst the challenges and hinting at a new chapter in their journey.