Two things you can do without any issues:

- uninstall old regular kernels, you aren't using them anyway
- remove old kdump files from the kernels that you don't use or don't exist on your system anymore.

You could alse remove old vzkernels, but I prefer to keep at least one vzkernel that is verified to work well for my systems (besides the active one, and that is not always the latest one).

After you remove either a kernel or a vzkernel, make sure grub has the correct kernel set as default (same as you do after a regular upgrade happens).

Anyway, even if you end up with 3 regular and 3 vzkernels, your /boot partition should be just about big enough.