I have 3 hosts (+1 for management with windows 7) and I would like to implement virtualisation on the Hyper-V platform. I downloaded Hyper-V Server 2008 R2.
Can the SCVMM be installed on windows 7 host? Does it have any other requirements?
SCVMM adds quite a few things over the basic Hyper-V manager app. You get things such as a storage library, so that you can create templates of VMs. Then rapidly deploy them out to your hosts. You get a very nice online P2V migration utility. You also get some batch management capabilities, such as being able to evacuate all VMs from a single host with a minimal set of clicks. The list goes on.
If by "virtual machine manager" you mean the Hyper-V Manager console, yes, it can be installed on Windows 7 (see here).
If you are instead referring to Microsoft's System Center Virtual Machine Manager, the centralized management system for Hyper-V hosts, then no, it will only run on Windows Server 2008 R2; but its management console will happily run on Windows 7.
You can find the complete system requirements here.