An SVI, or switched virtual interface, is a logical interface that is created on a Layer 3-capable device, such as a multilayer switch or a router. An SVI is associated with a VLAN and can be used to route traffic between different VLANs on the same device or across multiple devices. An SVI can also provide management access, security features, and quality of service (QoS) for the VLAN. An SVI is different from a physical interface, which is a port that connects to a physical device or network. A physical interface can be used for trunking, which is a method of carrying multiple VLANs over a single link, or for connecting to a single VLAN. An SVI is also different from a subinterface, which is a logical division of a physical interface that can be assigned to different VLANs.
Reference:
VLANs and Trunking - N10-008 CompTIA Network+ : 2.11
Switched Virtual Interfaces - N10-008 CompTIA Network+ : 2.22