It's a type-2 hypervisor, so its performance is often quite worse than KVM. Also, its storage drivers are quite wonky, so I/O is often choppy and underwhelming. It has often a better desktop experience, though, except when you run Linux with VirGL. Also, virtualbox cannot emulate other platforms; for instance, I often run AArch64 operating systems in full emulation using QEMU on my x86_64 workstation.
Yeah I didn’t say ideal. But if you are on Win/Mac (there are a lot of those people) and need a Linux box without much fuss it’s a good go-to solution.
No, KVM (and FreeBSD's bhyve) are kernel modules that for all intents and purposes transform the host kernel into a type 1 hypervisor itself. The hypervisor is not, like in the case of VrtualBox or VMware workstation, running on top of the kernel, it is the kernel itself. The VMs spawned on KVM or bhyve have potentially full access to any hardware capabilities exposed to them. You can passthrough PCI devices to KVM VMs, for instance, using VFIO; you can also fully expose the host CPU allowing nested virtualization
A "true" type-2 hypervisor does not require any modification to the host kernel; at max it ships with a kernel module or two to help implementing things such as custom networking or to get a speed boost kernel side. For instance, it's trivial to share USBs with a guest on the fly with VirtualBox, because the VM is basically a process running on the host, so it can just tap into the USB stack and expose the device to the VM. This is not so simple with KVM.
