SIM stands for "selected ion monitoring" - selected ions are continuously measured. This results in high sensitivity because an ion species can be summed over the entire time window of the measurement.
In SIM mode, no complete mass spectrum is measured.
Selected masses are constantly measured with high sensitivity. In addition, a mass spectrum is recorded at intervals.
Scan & XIC
XIC stands for “extracted ion chromatogram” - A complete mass spectrum is recorded continuously and the selected mass channels are extracted from it. The sensitivity for the mass channels is therefore lower than in SIM mode, but with the complete spectrum considerably more information is available - e.g. about peak purity.