An assignment of a contract transfers the benefits from one party to another party. However, the obligations and burdens of the contract are not transferred. The parties to the original contract do not change in an assignment - the contract remains in place. Only the person who is to receive the benefits from the contract is changed. For more information, read Assigning a contract.
A novation will create a new contractual relationship between two parties (ie the original contract is ended and a new one is formed). The new party effectively replaces one of the parties to the contract. All benefits and burdens under the original contract are transferred to this new party. For more information, read Novating a contract.