When you say a security officer, this normally means that the person is armed. A security guard will not be armed with a gun but may have another item that can also be used for protection. It means that there is more risk to be with a security officer as compared to a security guard.
There are also different tasks that they have to do. A security officer will be in charge of monitoring and inspecting the different access points of the place that is being secured. A security guard will usually be guarding the different entrance and exit points of the place to ensure that people who are not allowed to enter will not be allowed to enter.
A security officer is a step above a security guard in the sense that they have more training, experience, and skills. They also train and supervise the security guards. A security officer may be either ex-military or ex-police officers. This position is the type of respect that they command, as they may also be given the right to give orders to the security guards.
On the other hand, a security guard has numerous duties include issuing directives to other guards or planning work schedules for the guards. A security guard always has to maintain the same position at the same place. They could be guarding the entrance to the mall, banking hall, or home. They generally only require basic skills, and they do not necessarily need too much training.