According to the new definition within IPC-T-50M a microvia is a blind structure with a maximum aspect ratio of 1:1, terminating on a target land with a total depth of no more than 0.25mm measured from the structure’s capture land foil to the target land.
Blind via hole
It is a hole that runs from an outer layer to the inner layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology.The image shows a laser drilled blind via.
Buried via hole
This is a hole that runs between one or more inner layers. They are normally mechanically drilled.