LK is a Kernel designed for small systems typically used in embedded applications. It is a good alternative to commercial offerings like FreeRTOS or ThreadX. Such systems often have a very limited amount of ram, a fixed set of peripherals and a bounded set of tasks.
On the other hand, Zircon targets modern phones and modern personal computers with fast processors, non-trivial amounts of ram with arbitrary peripherals doing open ended computation.
Zircon inner constructs are based on LK but the layers above are new. For example, Zircon has the concept of a process but LK does not. However, a Zircon process is made of by LK-level constructs such as threads and memory.
More specifically, some the visible differences are:
Over time, even the low level constructs will change to accomodate the new requirements and to be a better fit with the rest of the system.