Are single displacement reactions exothermic?

Some are, some are not.

Whether a reaction is exothermic or endothermic has little to do with the type of reaction. It is a property of the specific chemicals of the reaction that determines whether the reaction is endo- or exothermic.

Specifically, it is the amount of energy released by breaking the bonds of the reactants, and the energy consumed in forming the bonds in the products, that determines whether it will be endo- or exothermic.

Think of a bond as a bottle of energy. In every molecule, the atoms are held together by these "bottles of energy" (potential energy). In order to form the products, though, those bonds must be broken, and the different bonds found in the products must be formed.

It's like opening the "bond-bottles" of the reactant bonds, and then pouring the energy from those bottles into the empty product bottles. If you have more energy in the reactant "bottles," then you will have some left over after filling all of the product bottles. That would be an exothermic reaction, because in the end, you release energy into the environment.

But if there isn't enough from the reactants to fill the product "bond-bottles", then you have to supply some energy from the surroundings in order to "fill all the bottles." That would be an endothermic reaction, because it absorbs energy from the surroundings.

Hope the analogy helps. :)