I'm using Linux Virtual Machines (Wheezy, Linux Vserver), each with a desktop environment including Firefox. 

If a Virtual Machine where to be compromised (e.g., *SQL injection*) so the kernel is hacked and control has been seized of the VM (not the host), it is possible to rebuild the VM, and move the data over to the rebuilt VM to fix the problem. 

Q: This may be a dumb question, but is it possible instead as an alternative to copy some files into the compromised VM (e.g., a clean version of everything in the `/boot` partition)? The VMs are pretty tightly locked down to begin with.

The argument here being this might be quite a bit faster than rebuilding the VM. Or, the "right answer" may be "nope, you need to rebuild the VM to be sure it hasn't been compromised". If the alternative approach were worth considering, what would need to be overwritten and replaced to have a "fresh" kernel?