A: Service packs, as you probably know, generally exist as a rollup of a series of hotfixes over time. However, you don't have to wait for the next service pack to be released to add hotfixes to your Hyper-V servers. Some hotfixes might fix problems that simply can't wait.
An important website to keep in your bookmarks list is Microsoft's "Hyper-V Update List for Windows Server 2008 R2." This website is Microsoft's official list of Hyper-V hotfixes for Windows Server 2008 R2. Some are considered required, while others should only be installed if you are experiencing the specific problem the hotfix intends to fix.
Pay particular attention to the comments at the bottom of this page. In those comments are additional hotfixes that haven't yet been added to the official list.