The primary difference between a nova and a supernova is the strength of the explosion. Novas and supernovas are similar in that they both produce a large release of energy through the exchange of gases between two stars. The stars involved include a normal star and a white dwarf, which is a star that is dying.
A nova occurs when the normal star and the white dwarf orbit each other closely enough for the white dwarf to draw hydrogen gas off the star to create a release of energy. The gas is compressed on the surface of the white dwarf through strong gravitational forces and fuses into helium, which causes the explosion. Visually, the white dwarf becomes suddenly brighter and slowly fades away.
The type of supernova that is similar to a nova goes through the same process, but under the right circumstances, the hydrogen gas compresses more densely. When it fuses on the surface of the white dwarf to become helium, it creates a much larger explosion. The explosion can be so violent that the white dwarf may be completely destroyed.
The second type of supernova happens between a white dwarf and a much larger star. The type of star that creates a supernova has a mass that is eight times larger than the sun. The white dwarf draws the hydrogen gas into its core to create helium, but when the hydrogen runs out, the helium fuses into carbon. From there, a number of fusions occur until the core is primarily made of iron. As the pressure builds further, the iron begins to fuse, extracting the heat and the electrons from the core. This causes the core to collapse and the outer layers of the star to explode, creating a supernova.