Lava and pumice are of both volcanic origins. They are however slightly different. They differ by color, density, element composition, vesicle size, and water retention capacity.
Pumice is lighter because of the air mixed in it, while lava rock is denser. Lava rock also has bigger vesicles and thicker vesicle walls than pumice.
Lava is a type of scoria rock and it is a molten rock released from a volcano during an eruption. The subsequent rock solidifies and cools to result in lava rock, while pumice is a mixture of volcanic ash and rock.
Lava rock, just like pumice, is great for enhancing drainage and they are great for succulents. However, pumice comes in a better size and structure for your plant use.