Qubes os vs KVM on Linux(This is not war!)
Cons of Qubes os
Security by separation vms, Qubes os is most secure os.
Unless bug is exist on Xen, threats can not attack to dom0.
If user must copy data to dom0 from vm, this process is made very complexly, fail of user is been suppressed default.
Multi templates can use at the same time without dual boot(But Qubes os supports to Linux and Windrows only).
User can use disp-vm, all data on disp-vm are delete with shutdown, SSD usage suppresses.
Pros of Qubes OS
Qubes os eat a lot of RAM than KVM, it needs to larger RAM.
Qubes os is Xen distribution, so usability is difficult for light user.
Secondly storage uses difficultly on Qubes os, if standalone vm installs difficulty in HDD or USB, user needs to install neither Linux nor Windows os, SSD usage becomes bloated.
System requirement of Qubes os is strict, for example there are a few of PC with PS/2 connection today, Qubes os can run only on limited PC.
Functions of Qubes os dependents to systemd services, user does not have freedom of choice init.
Neither pros nor cons between Qubes os and KVM
Separation between vm and other vm.
Qubes os not yet supports to Wayland, but vms are separating about each other, X11 server can not know to task of other vm.
KVM supports to Wayland, Wayland is separation running apps, this security is same to design of Qubes os.
Text or file copies to other vm, it supports both Qubes os and KVM(But Qubes os supports to only Linux and Windows).
User can use full disk encryption.
Cons of KVM
KVM runs on Linux, and Virt-Manager can manage for KVM, so using KVM is more easily than Qubes os.
KVM users are larger than Qubes os users, so documents of KVM are larger than documents of Qubes os.
If guest os runs on KVM, security of host is safety than VirtualBox.
If user uses Alpine Linux as KVM host, Alpine can install to RAM and run from RAM, guest os of KVM on Alpine is perfectly disposable(Alpine supports to LBU, setting and date on guest vm are able to kept).
User can select lightweight distribution as if VOID(Minimal RAM is 96MB) or Alpine(Minimal RAM is 128MB), user cun use KVM on old and a few of RAM PC(Minimal RAM of Qubes os is 6GB).
KVM supports to os neither Linux nor Windows as guest, for example KVM can copy file to Haiku from FreeBSD, Qubes os can not this.
KVM can run without systemd, user can be free to choice host distribution.
User can install guest os in secondly storage same to in first storage.
Pros of KVM
KVM runs only on Linux, so user must select only Linux as host.
Linux is not designed for security, so not secure codes are existing a lot of on Linux kernel, threats can use their codes for attack to host.
Linux is written by C language, C language is not memory safely, this is attack surface.
KVM is not designed for security, so if user use KVM for security, user must set KVM myself.
KVM runs on Linux, so host Linux is not safely than dom0 of Qubes os(But host Linux can use guest vm for network access like as router).
KVM separates vms each other, but separation between host and guest is inferior to Qubes os.
KVM is inferior to VirtualBox about security of guest vm(KVM does not support disp-vm and snapshot, if guest vm compromised once, user must delete all of vm data).
KVM can guard from attack to host through guest vm, but KVM is weak defense from direct attack to host.
User must upgrade host Linux for security kept, but host Linux must be online for upgrade, this is not safely(But host Linux can access to repository through guest vm, or it can use Tor network for upgrade, host Linux can keep safety).
I think if KVM uses as alternative Qubes os, user choices Alpine Linux as host and enable LBU, and OpenBSD(Minimal RAM is 32MB) installs in KVM as router vm, and other lightweight os installs as guest.
For example KVM run on Alpine Linux, OpenBSD and VOID install as guest, requirement of minimal RAM is less than 512MB.
Minimal RAM of Qubes os is 6GB, and recommend RAM is 16GB.
So if user has PC of less than 4GB RAM, user can use KVM on Alpine Linux as alternative Qubes os.