A Leviathan class hive ship is the largest Tyranid ship made. They are the center of a hive fleet and function as the main producer of Tyranids. Each is capable of holding many thousands of invaders. They are also the ship that draws up the resources of a planet after a successful invasion and "devours" the material to form new creatures and ships.
Leviathan class ships are capable of having five major weapons arrays, one on its prow, three on its sides, and one on its "thorax." They are slow and have problems meeting enemies. They are a "synapse" creature and thus operate as the mind of the fleet, and are often carriers as well, able to pump out astounding amounts of living creatures; since smaller Tyranid spacecraft do not need to rearm and refuel (they are essentially kamikaze troopers), the Hive Ships can send out a quite literally endless stream of craft.
Hive Ships are the biggest known ships in the Tyranid fleets, and are roughly the same size as Imperial battleships. However, some Hive Ships have evolved to grow inordinately big, growing to sizes so astounding that they dwarf even battleships.