Physical memory is not fungible like that across VMs. So, you can expect stuff loaded into memory to stay there unless your kernel inside the VM decides it not to.
No, it's. VirtIO's balooning device can "inflate" to pseudo-allocate memory on a VM to free physical memory for other hosts.
Moreover, even if your files in memory, you cannot reserve "memory controller bandwidth". A VM using tons of memory bandwidth or a couple of cores in the same NUMA node with your VM will inevitably cause some traffic and slow you down.
