Contrary to what you might have seen in movies, if you jumped into lava you would not sink. Lava is nothing like water. Lava is a term used for molten rock on the surface of the earth and is incredibly dense, over 3 times the density of water.
Water has a density of 1000 kg/m3 and Lava has a density of 3100 kg/m3
Humans have a density of around 1010 kg/m3 which is very similar to water and this is the reason that we can float and not sink in water. However lava has a density much higher than our own so we would not sink or float, we would be stuck on the surface.
Viscosity is also a factor, this determines how much resistance something gives to things flowing through it. For example if you drop something in water, the water particles will move aside to let the object pass through. However this doesn’t happen with lava as its viscosity is between 100,000 to 1,100,000 times the amount of water! Lava will not let things flow through it easily and is a thick solid liquid.
However you will not get off lightly if you did descide to jump into a volcano, while you would not sink, you would most likely break your bones on impact and swiftly burn to death on the surface or the water in your body would boil and you would explode.