Kind of a vague question, but I take it you mean OS-level hardening, which should be fine with CIS hardening.

In a virtualized environment, there are many security layers to take care of: network access, storage, api control, identity access, cluster config, backups, etc.

Firstly, Xen is considered by secure by Qubes — but that’s mainly the security of the hypervisor and virtualization system itself. They make a very compelling argument that escaping a Xen based virtual machine is going to be more difficult than a KVM virtual machine.

But threat model matters a lot. Qubes aims to be the most secure OS ever, for use cases like high profile journalists or other people who absolutely need security, because they will literally get killed without it.

Amazon moved to KVM because, despite the security trade off’s, it’s “good enough” for their usecase, and KVM is easier to manage because it’s in the Linux kernel itself, meaning you get it if you install Linux on a machine.

In addition to that, security is about more than just the hypervisor. You noted that Promox is Debian, and XCP-NG is Centos or a RHEL rebuild similar to Rocky/Alma, I think. I’ll get to this later.

Xen (and by extension XCP-NG) was better known for security whilst KVM (and thus Proxmox)

I did some research on this, and was planning to make a blogpost and never got around to making it. But I still have the draft saved.

Name	Summary	Full Article	Notes
Performance Evaluation and Comparison of Hypervisors in a Multi-Cloud Environment	Compares WSL (kind of Hyper-V), VirtualBox, and VMWare-Workstation.	springer.com, html	Not honest comparison, since WSL is likely using inferior drivers for filesystem access, to promote integration with host.
Performance Overhead Among Three Hypervisors: An Experimental Study using Hadoop Benchmarks	Compares Xen, KVM, and an unnamed commercial hypervisor, simply referred to as CVM.	pdf
Hypervisors Comparison and Their Performance Testing (2018)	Compares Hyper-V, XenServer, and vSphere	springer.com, html
Performance comparison between hypervisor- and container-based virtualizations for cloud users (2017)	Compares xen, native, and docker. Docker and native have neglible performance differences.	ieee, html
Hypervisors vs. Lightweight Virtualization: A Performance Comparison (2015)	Docker vs LXC vs Native vs KVM. Containers have near identical performance, KVM is only slightly slower.	ieee, html
A component-based performance comparison of four hypervisors (2015)	Hyper-V vs KVM vs vSphere vs XEN.	ieee, html
Virtualization Costs: Benchmarking Containers and Virtual Machines Against Bare-Metal (2021)	VMWare workstation vs KVM vs XEn	springer, html	Most rigorous and in depth on the list. Workstation, not esxi is tested.

The short version is: it depends, and they can fluctuate slightly on certain tasks, but they are mostly the same in performance.

default PROXMOX and XCP-NG installations.

What do you mean by hardening? If you are talking about hardening the management operating system (Proxmox’s Debian or XCP’s RHEL-like), or the hypervisor itself?

I agree with the other poster about CIS hardening and generally hardening the base operating system used. But I will note that XCP-NG is more designed to be an “appliance” and you’re not really supposed to touch it. I wouldn’t be suprised if it’s immutable nowadays.

For the hypervisor itself, it depends on how secure you want things, but I’ve heard that at Microsoft Azure datacenters, they disable hyperthreading because it becomes a security risk. In fact, Spectre/Meltdown can be mitigated by disabling hyper threading. Of course, their are other ways to mitigate those two vulnerabilities, but by disabling hyper threading, you can eliminate that entire class of vulnerabilities — at the cost of performance.