I get a beautiful render using Karma CPU but with Karma XPu that's a different story. Why are these renders so different? Karma XPU isn't compatible with such elements yet?

I can vaguely make out the words "principledshader" and "pyroadvancedshader" in your screenshot, both of which are Mantra materials. Unfortunately, XPU doesn't support Mantra materials. Only Karma and MaterialX materials can be used.

In addition to @jsmack's excellent point about materials, it's also worth mentioning that Karma CPU and XPU are entirely different engines. It's not like RenderMan, where XPU is presented as the "interactive" flavor of RenderMan. They can give very similar results, and there will be some similarities because of shared code.

But Karma XPU is not meant to match pixel-for-pixel with Karma CPU. Karma XPU should give pixel-for-pixel results regardless of the devices it uses. XPU can run on pure CPU, pure GPU, or mix of CPU and GPU devices.

Ah! Thank you both. I forgot about Mantra materials not compatible with XPU.
Any tip or good tutorial about rendering volumes with Karma XPU?
Thanks

Is there an equivalent to the pyro advanced shader with MTLX? I need to bind attributes to smoke, fire, secondary fire.
Should I do as in this Rohan Dalvi tutorial ? https://www.youtube.com/watch?v=KuI9B1MA6do&t=12s [www.youtube.com] . But he says it only work with Karma CPU...

Grendizer
Is there an equivalent to the pyro advanced shader with MTLX? I need to bind attributes to smoke, fire, secondary fire.
Should I do as in this Rohan Dalvi tutorial ? https://www.youtube.com/watch?v=KuI9B1MA6do&t=12s [www.youtube.com] . But he says it only work with Karma CPU...

The XPU Pyro Preview shader is the way to go currently: https://www.sidefx.com/docs/houdini/nodes/vop/kma_pyropreview.html [www.sidefx.com]