Proxmox kann Out Of The Box leider nur full Backups. Um diesen Umstand zu verbessern gibt aus der Community einen Patch
https://github.com/ayufan/pve-patches

Es muss der richtige Patch für die richtige PVE Version installiert werden. Es kommt auch oft vor das der Patch für die neuerste Version nicht verfügbar ist. Dazu geht es weiter unten eine Info.

Anzeigen der nötigen Versions Infos

dpkg -l  | grep "pve-manager\|qemu-server"
ii  pve-manager                          6.2-4                          amd64        Proxmox Virtual Environment Management Tools
ii  qemu-server                          6.2-4                          amd64        Qemu Server Tools

Installieren des Patches

git clone https://github.com/ayufan/pve-patches
cd pve-patches
bash pve-6.2-4-diff-backup-addon test
bash pve-6.2-4-diff-backup-addon apply
Proxmox VE 6.2-4 - differential backup support, v3
Kamil Trzcinski, http://ayufan.eu/, ayufan@ayufan.eu

checking file usr/share/perl5/PVE/LXC/Create.pm
checking file usr/share/perl5/PVE/QemuServer.pm
Hunk #1 succeeded at 5493 (offset -128 lines).
Hunk #2 succeeded at 6139 (offset -126 lines).
Hunk #3 succeeded at 6349 (offset -126 lines).
checking file usr/share/perl5/PVE/Storage.pm
checking file usr/share/perl5/PVE/Storage/Plugin.pm
checking file usr/share/perl5/PVE/VZDump.pm
checking file usr/share/perl5/PVE/VZDump/Common.pm
checking file usr/share/perl5/PVE/VZDump/LXC.pm
Hunk #1 succeeded at 333 (offset 16 lines).
Hunk #2 succeeded at 435 (offset 23 lines).
checking file usr/share/perl5/PVE/VZDump/QemuServer.pm
Hunk #1 succeeded at 241 (offset -2 lines).
Hunk #2 succeeded at 249 (offset -2 lines).
Hunk #3 succeeded at 473 (offset 1 line).
Hunk #4 succeeded at 548 (offset 1 line).
checking file usr/share/pve-manager/js/pvemanagerlib.js
$ patch_apply /
patching file usr/share/perl5/PVE/LXC/Create.pm
patching file usr/share/perl5/PVE/QemuServer.pm
Hunk #1 succeeded at 5493 (offset -128 lines).
Hunk #2 succeeded at 6139 (offset -126 lines).
Hunk #3 succeeded at 6349 (offset -126 lines).
patching file usr/share/perl5/PVE/Storage.pm
patching file usr/share/perl5/PVE/Storage/Plugin.pm
patching file usr/share/perl5/PVE/VZDump.pm
patching file usr/share/perl5/PVE/VZDump/Common.pm
patching file usr/share/perl5/PVE/VZDump/LXC.pm
Hunk #1 succeeded at 333 (offset 16 lines).
Hunk #2 succeeded at 435 (offset 23 lines).
patching file usr/share/perl5/PVE/VZDump/QemuServer.pm
Hunk #1 succeeded at 241 (offset -2 lines).
Hunk #2 succeeded at 249 (offset -2 lines).
Hunk #3 succeeded at 473 (offset 1 line).
Hunk #4 succeeded at 548 (offset 1 line).
patching file usr/share/pve-manager/js/pvemanagerlib.js
APPLIED

$ service pveproxy restart
$ service pvedaemon restart