1) The Hypervisor

And we are back to the Hypervisor.  So you are a XenServer shop right, man don't you love XenMotion, its just like vMotion but with a bit of Xen to it.  Oh no, not a XenServer shop... oh well then don't you love HyperV?  Don't you love that they have "vMotion" too, sure you can only do one job at a time to one destination and that destination or host cannot do anything else during that time (seriously WTF Microsoft!).  (Composing myself...)  Sorry, okay, most likely you are running ESXi, at least I really hope so because all the other hypervisors are... um lacking... (and I said I wouldn't be bias... okay to be fair on the Hypervisor front I am very bias, you'll just have to let this one go.)
Word on the street is that 80% of XenDesktop implementations are done on ESXi.  I consider this a bonus for XenDesktop.  I mean if you think about Citrix as a hole, they have always been great creating products that complement another product.  XenApp, Citrix's bread and butter, is a compliment to Microsoft Terminal Services.  To say that 80% of XenDesktops are implemented on ESXi is perfectly in line with Citrix's business model and I think that's a great thing.  I also think its great that XenDesktop will install on HyperV.  Yes, I think its an inferior product to both ESXi and XenServer, but who cares what I think, if HyperV works great for you, guess what, XenDesktop will work great on that.  What a smart move for Citrix to make their product work on any HyperVisor.  I do like XenServer more than HyperV, but I think they both don't hold a candle to ESXi.
The first layer of any virtualization solution is the Hypervisor.  The main HVs on the market are HyperV, XenServer and ESXi.  I'm focusing on ESXi because that's the market.  However, from a price stand point if you are going to deploy XenDesktop and you are in an ESXi shop you'll need to run two Hypervisors, ESXi for your servers and XS for your desktops.  Why?  Because although you get ESXi free when you buy VMWare View, you don't get it free when you buy XenDesktop.  So you'd need to purchase it seperately.  Most shops just have a seperate XS cluster for just the desktops.  As there isn't a ton of work to maintain the Hypervisor layer I see this as a passable approach, however I'd recommend ESXi, period.  Also, VMWare does provide a special license for running virtual desktops only, this is to help the non-View VDI customer into using the best Hypervisor (I did mention I'm bias on this right?)

What is a Hypervisor?
I'm still amazed that some businesses haven't moved all (or at least a good portion) of their servers to the virtual realm yet.  However, this is still the case so I still need to explain what a hypervisor is.  A hypervisor is an extremely small OS, that acts as a go between between your hosts physical components and the virtual machines you will place on that host.  So you install ESXi locally on the physical server, then a virtual machine of Windows 2008 runs on that host.  Instead of Windows 2008 interacting directly with the physical hardware, it interacts with virtual hardware created by the hypervisor.  When you look on your VM, you will see drivers that have nothing to do with your physical hardware, instead these drivers are written to work with the hypervisor's virtualized hardware.  This separation of physical hardware to virtual hardware is what makes it possible to do all sorts of wizardry on theses virtual servers.  You can now share resources, CPU, RAM, NIC, etc.  You can also move servers between physical hardware while they are running, with no downtime (this is called vMotion); you can do the same with storage (svMotion).  There are numerous benefits to virtualization, most are awesome bells and whistle that the IT person loves, however it comes bundled at a lower CAPEX and OPEX cost than typical hardware so your CFO and boss will be happy with this solution as well.

Honestly, Mr/Mrs. IT person reading this, if you want to get the greatest pat on the back ever, push to virtualize your server infrastructure.  Start with ESXi so it doesn't cost you a penny, prove that you can run a bunch of virtual servers on one physical server then once you have your boss hooked, go buy the real product and get the awesome bells and whistles.  "Once you go virtual you will never go back" I think Wesley Snipes said that...

Back on track here: a Hypervisor is required for VDI, it puts the V, as in Virtual, in VDI, so we need that, I'm choosing ESXi.  I don't want to focus on the Hypervisor too much becuase that is a conversation all in itself.  Go out and get your VCP, so you can be all smart and certified.  Oh, and no complaining that VMWare requires you to go to a course to get your VCP.  I LOVE that they require this, unlike the textbook MCSEs out there, you need real world hands on experience to get a VCP... I digress.

I digress a lot... did anyone notice that.

I love lamp.