Hwang's crime, according to the South Korean media, was to have developed a number of ideas to improve agricultural production.
These ideas were seen by the regime as an attempt to undermine Kim.
Both men were executed by an anti-aircraft gun at a military academy in Pyongyang.
The North Korean regime is especially paranoid in recent weeks after a senior offiicial at the London embassy defected to South Korea along with his wife and children.
North Korea rarely announces purges or executions, although state media confirmed execution of Kim's uncle and the man widely considered the second most powerful man in the country, Jang Song Thaek, in 2012 for factionalism and crimes damaging to the economy.
A former defence minister, Hyun Yong Chol, is also believed to have been executed last year for treason, according to the South's spy agency.
The JoongAng Ilbo said the two men were executed by anti-aircraft gun at a military academy in Pyongyang.