I've recently spend some time playing around with the xv6 kernel (more specifically a 64bit port of it), one problem is that the amount of work necessary to get

I've recently spend some time playing around with the xv6 kernel (more specifically a 64bit port of it), one problem is that the amount of work necessary to get a reasonable clean hardware abstraction for a x86 system is fairly complicated. For example for a full acpi implementation, you either have to write ~50 admittedly short glue functions for ACPICA or implement acpi table reading and an AML interpreter yourself. Unfortunately if you want to use modern features of PCI, there is no way around using acpi. That being said, there are several such "modular" kernels implemented on top of Xen. For example Mirage is a framework for creating free standing Ocaml applications on top of Xen.