K means unlocked and therefore overclockable.  However, I do not think you'll find the XPS models support overclocking -- you need an Alienware system to have that option.  The fact that Dell ships the K models likely has more to do with component shortages than anything else. XMP is  memory technology -- it has nothing to do with the CPU.  The XPS models (and even most Alienwares) do not support XMP.  
I´m looking at an XPS 15 and I see there is an option for an i9-12900HK processor. What is that "K" going to do for me?  Does it mean I can overclock it?  Does it come with XMP? Thanks for your help! Tiny Tech
i9-12900HK Processor - Why the "K"?