dinamic fixed or kinematic

btRigidBody is the main class for rigid body objects.
It is derived from btCollisionObject, so it keeps a pointer to a btCollisionShape. It is recommended for performance and memory use to share btCollisionShape objects whenever possible. There are 3 types of rigid bodies:

A) Dynamic rigid bodies, with positive mass. Motion is controlled by rigid body dynamics.
B) Fixed objects with zero mass. They are not moving (basically collision objects)
C) Kinematic objects, which are objects without mass, but the user can move them. There is on-way interaction, and Bullet calculates a velocity based on the timestep and previous and current world transform. Bullet automatically deactivates dynamic rigid bodies, when the velocity is below a threshold for a given time. Deactivated (sleeping) rigid bodies don't take any processing time, except a minor broadphase collision detection impact (to allow active objects to activate/wake up sleeping objects)