Difference between Delcam Robot powermill and Kuka CNC

Not sure about the differences in programming, but for sure the output of Powermill Robot is a .SRC file to be executed directly on the robot - meaning no additional software needed.
May be (not sure) you will need the Kuka.CNC package in terms to feed the files for milling if/when they are too much for the memory of the controller

Here's some info that I hope will be helpful, but with some caveats first:
1.) We have PowerMill with Robot Interface and use it with our KRC4 KR210 R2700 robot; I evaluated the options for our robot setup and made the purchasing decisions but the setup is normally programmed and run by other Engineers in my company. When we got the robot setup (about 3-4 years ago), PowerMill with its Robot Interface appeared to be bar far the most developed and capable package. There are a number of other packages now out there for robot machining that we haven't tried.
2.) We've looked into Kuka CNC when it came out (after we got PowerMill) but did not buy it
Anyway...
PowerMill is a CAM package that develops tool paths for machining, which it can output as g-code for conventional CNC mills (robot controllers don't understand g-code). The Robot interface part of PowerMill is a post processor that converts g-code to code that various robot controllers can understand and run. It also allows you to run simulations of your machining paths with your robot cell setup to make sure you won't crash the robot during machining.

KUKA CNC gives the robot controller the native ability to read g-code. As a result, for a Kuka CNC equipped robot there is no need for a post processor like PRI to convert g-code to robot code; A Kuka CNC equipped robot should be able to read any g-code file and follow the paths and commands within it, as long as they paths are something the robot can physically do of course. On the other hand, a post-processor that allows you run tool paths with a simulated robot cell to check for crashes is a useful tool.

However, KUKA CNC is not a g-code generator so even if you had a robot with it, you would still need a CAM package to develop your machining paths and write them to g-code if you want to machine with your robot. With Kuka CNC you should be able to open g-code files created by pretty much any g-code generating CAM package, rather than being locked into PowerMill (which is not necessarily a bad thing; PowerMill is very capable).

In summary:
PowerMill with Robot Interface allows you to:
Develop tool paths for machining
Check whether tool paths might lead to a collision (or a singularity)
Convert g-code to robot code so you can run it on your robot

Kuka CNC allows;
Your robot to read g-code and run it
BUT:
Software to develop tool paths and write to g-code is still required
Software to simulate the robot following the tool paths in your cell setup is not required, but is very useful

The ability of Kuka CNC to allow a robot to natively read g-code offers a few other advantages, but that's probably another subject.

Forgive my ignorance.

I've used PowerMILL but know nothing of Kuka CNC.

Is it anything like KRL or CamROB?

Quote from Sculptor

Forgive my ignorance.

I've used PowerMILL but know nothing of Kuka CNC.

Is it anything like KRL or CamROB?

i think camrob is for krc2 and kuka.cnc for krc4.if im not mistaken.
we have these kind of system. good thing about using kuka.cnc youll get get a better result compare to krl.
coz gcode=interpolates. krl=approximates.
and with kuka.cnc youll get almost unlimited memory space for code since it uses drive c:.
you can used your powermill to generate gcodes. you need someone to create a postprocessor.
and to configure the kuka.cnc on your robot specially if you have external axis.

does running of a program from delcam robot power mill requires kuka cnc to be installed on a kuka robot.
.no. it doesnt requires kuka.cnc. you can write a full krl program.
What are the formats used for programming.
.without kuka.cnc it should be 2 files. .src and .dat