Standby, or active/passive operation is not load-balanced at all. Each LUN is accessed using a specific path. The route changes only when the assigned path becomes unavailable. At that point, the route fails over to the standby path. If the host is connected to multiple LUNs, each LUN can be assigned to a different path.
Round-robin load-balancing is the simplest method in terms of true load- balancing. I/Os are distributed to each available path in a rotating manner. This can be done per-Exchange or per-frame.