I have an Ubuntu 14.04.3 (with linux-generic-lts-vivid (3.19) kernel) 64-bit host, and a VirtualBox guest with the same OS and configuration.
The host has an AMD A10-7800 CPU.
If I enable the KVM paravirtualization interface in VirtualBox, do I need to change anything in the guest in order to take advantage of it?
From Virtualbox manual:
Most mainstream, modern operating systems, including Windows and Linux, ship with support for one or more paravirtualization interfaces. Hence, there is typically no need to install additional software in the guest (including VirtualBox Guest Additions) to make use of this feature.
VirtualBox provides the following interfaces:
Minimal: Announces the presence of a virtualized environment. Additionally, reports the TSC and APIC frequency to the guest operating system. This provider is mandatory for running any Mac OS X guests.
KVM: Presents a Linux KVM hypervisor interface which is recognized by Linux kernels starting with version 2.6.25. VirtualBox's implementation currently supports paravirtualized clocks and SMP spinlocks. This provider is recommended for Linux guests.
Hyper-V: Presents a Microsoft Hyper-V hypervisor interface which is recognized by Windows 7 and newer operating systems. VirtualBox's implementation currently supports paravirtualized clocks, APIC frequency reporting, guest crash reporting and relaxed timer checks. This provider is recommended for Windows guests.
Ubuntu kernels are compiled with KVM guest support, as you can see in the output of grep CONFIG_KVM_GUEST /boot/config-*, so there's no need to change anything in the guest system.
I think you should install Guest Additions. I didn't run any benchmark but I felt the guest system more fluid and responsive with Guest Additions & KVM when compared to KVM alone. Also, without Guest Additions you won't be able to access shared folders or copy/paste between host and guest.
