Hi,
I need to have a look at the Hadoop-specialized Cloudera distribution and thus would need to set up some virtualization system to install it.
What would be the easiest engine to set up to check Cloudera on a Slackware64-current system among the followings :

VMware Workstation
VirtualBox
Xen
KVM
Qemu

I don't need fancy features like FC support, OpenGL or snapshots, just network support basically, as I'll need to set up two or three VM and make them communicate.
Thanks!

i would say qemu+kvm

You'll need qemu from slackbuilds.org for all the following suggestions. I'd recommend that you install spice prior to qemu.

I'd recommend virt-manager from Slackbuilds.org (and all of its dependencies).

I use it all the time to create multiple VMs that talk to each other. I've got mongoDB with SSL support building on a Centos VM on a nearby Slackware64 machine while I'm writing this.

AQemu and dependencies from Slackbuilds.org is also a good choice. I started using that one when I first attempted virtualization.

I started my virtualisation experiments with VirtualBox because of the quick and easy graphical frontend, but grew tired of having to compile the associated kernel modules with every kernel upgrade.
I much prefer qemu+kvm now. Once set up, it has proved very reliable and easy to maintain. I do not have sufficient need to build virt-manager, but if I was starting from scratch it would things much easier.
Xen has a primary goal of virtualising Linux systems, which is not my focus, so I have not explored it. It has needed custom kernel compilation in the past.
VMware I have used on Windows. It is very good, the granddaddy to other virtualisation attempts, but has a commercial focus.

Thank you all!
There seems to be a consensus here so I'll go for qemu + kvm.