this question will open a debate about what is better than other but anyway.....
The answer for your first question is well explain in server fault section
well in my opinion VirtualBox is the guy for you.....
well kvm is more complex....KVM (for Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V). It consists of a loadable kernel module, kvm.ko, that provides the core virtualization infrastructure and a processor specific module, kvm-intel.ko or kvm-amd.ko. KVM also requires a modified QEMU although work is underway to get the required changes upstream.
Using KVM, one can run multiple virtual machines running unmodified Linux or Windows images. Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc.
The kernel component of KVM is included in mainline Linux, as of 2.6.20.
KVM is open source software.
Check that your CPU supports hardware virtualization
To run KVM, you need a processor that supports hardware virtualization. Intel and AMD both have developed extensions for their processors, deemed respectively Intel VT-x (code name Vanderpool) and AMD-V (code name Pacifica). To see if your processor supports one of these, you can review the output from this command:
egrep '(vmx|svm)' /proc/cpuinfo
egrep -c '(vmx|svm)' /proc/cpuinfo
If 0 it means that your CPU doesn't support hardware virtualization.
If 1 (or more) it does - but you still need to make sure that virtualization is enabled in the BIOS.
Yes KVM have a GUI :
apt-get install virt-manager