A hypernova, also known as a collapsar (short for collapsed star), is an extremely energetic supernova.
Difference between a Supernova and Hypernova
In a supernova, a star shears off its outer matter but leaves a new star at its centre, often a neutron star. In a hypernova, the force of the explosion tears the inner star apart too.
How do Hypernova's Occur?
The cause of Hypernovas is still unclear but one theory is that a massive star rotating at a very high speed or encased in a powerful magnetic ﬁeld explodes, ripping apart the inner core. Alternatively, a hypernova could be the result of two stars in a binary system colliding with each other, merging into one gigantic mass and subsequently exploding.
When do Hypernova's occur?
Hypernovas occur in stars with a mass greater than 30 times that of our Sun. Like in a supernova, as the star runs out of fuel it can no longer support itself under its own gravity. It collapses and subsequently explodes, sending out matter in all directions. This releases more energy in seconds than our Sun will in its entire 10 billion-year lifetime.
Power of a Hypernova
A hypernova explosion, the possible source of powerful gamma-ray bursts, is about 100 times more energetic than a supernova explosion and is thus the most energetic event known in the Universe other than the Big Bang.
Note: For a much better explanation about Hypernovas please look at this.