VirtualBox + KVM: Linux Guest Additions: Kernel 6.12 Support – What You Need to Know
From Linux kernel 6.12, KVM initializes virtualization when its module loads, so VirtualBox VMs fail to start with VERR_SVM_IN_USE. There are two fixes: add kvm.enable_virt_at_load=0 to your kernel command line and reboot, or unload the KVM modules before you start your VM.