Kernel is the core of Linux. Think of it as the drivers for all the hardware that runs on the device.
Newer kernel (drivers), tend to contain increased performance, fixes and various other changes.
3.x is extremely old and lacks some of the new features available to 4.9.
We do not officially support OMV at this time. However, DietPi offers the most of the features with its scripts, and in much lighter installation.