With Hyper-V clustering you have a pool of Hyper-V servers (2+) all attached to the same set of network storage, so that the LUNS are available to all servers in the Hyper-V cluster. You must have network storage for this setup. With Hyper-V Live Migration, you can move a running VM from one Hyper-V host to another. This allows the workload from one server to shift over to another if the sever fails. This gives you physical redundancy if your remaining servers are capable of handling the load of the additional VMs. This setup does not protect you from corruption of the VM's OS and applications themselves. (See http://technet.microsoft.com/en-us/library/dd446679(WS.10).aspx for details about this setup.)
SQL has its own redundancy available with several different clustering options. You can either do traditional active/passive clustering, with an active node and one or more passive nodes. This setup requires a shared disk between the servers and is only mounted on the active node. SQL also supports serveral types of replication which allows multiple active nodes. This method does not require shared storage and keeps a separate copy of the database on each server. (See http://msdn.microsoft.com/en-us/library/ee523927(v=sql.100).aspx for SQL 2008 high availability options)
Clustering at the SQL level protects from a failed OS or application on an individual node, allowing automatic failover in that scenario. If each instance is on a different Hyper-V server you are also protected from hardware failures. Additionally, some of the clustering methods for SQL server protect against corruption of the database on individual nodes. Using a Hyper-V cluster and only a single instance of the SQL server does not protect you from OS/software failure in the VM. If downtime is not a big issue you could restore from a VM snapshot in a short amount of time.
Forgot the IIS load balancing part.
For load balancing IIS you can use Window Network Load Balancing, which creates a virtual IP that is shared between both hosts. (See http://technet.microsoft.com/en-us/library/cc770689(v=ws.10).aspx)
The same rules apply with the IIS server as the SQL server for whether Hyper-V clustering or NLB is the right option. In addition for your other VMs, unless they are clustered/load balanced as well, they are not protected from an issue with the Hyper-V host without Hyper-V clustering.