kadb is a bootable kernel debugger with the same general syntax as adb. See kadb(1M) for the exceptions. One advantage of kadb is that the user can set breakpoints and when the breakpoint is reached, examine data or step through the execution of kernel code.
If the system is booted with kadb -d, values for variables in the core kernel can be set, but values for loadable modules would have to be set when the module was actually loaded.
See “Debugging” in Writing Device Drivers for a brief tutorial on using the kadb command.