In the currently shipping version of Hyper-V – that is, that one that ships with Windows Server 2008 R2 RTM – dynamically adding resources is not supported. Coming in Service Pack 1 is a feature that Microsoft is calling Dynamic Memory, which adjusts the amount of memory available to virtual machines depending on the needs of each virtual machine. It should be noted that Dynamic Memory is, well, a bit more dynamic that a simply hot add operation. Dynamic Memory allows Hyper-V to constantly monitor memory needs inside virtual machines and then add or remove RAM as necessary to support expanding and contracting workloads. It should be noted that, although Dynamic Memory can add additional RAM to a running VM, it can't, in reality, remove said RAM once it's no longer needed, so the feature relies on some trickery to make the "removed" RAM no longer available to the VM while returning said RAM for other use.
Dynamic Memory is supported on a number of Windows operating systems, including Windows 7 Ultimate and Enterprise and Windows Server 2008 (and R2) Web, Standard, Enterprise and Data Center. Microsoft also plans to support Dynamic Memory for VMs running Windows Server 2003 (and R2) Web, Standard, Enterprise and Data Center.
Note: Dynamic Memory is not akin to VMware's hot adding capability. Once memory values are set for the VM, Dynamic Memory can work within the range, but you can't just go to the VM properties and add a bunch of RAM.