What's the technical difference between the following two backup and restore technologies?
(1) "Image" backup solution such as "dd" command on Unix?
(2) "Snapshot" backup solution such as EMC's SnapView?
Any performance differences between these two technologies?
These are two different concepts. An image backup refers to a semi-traditional backup where data is being backed up from disk to (probably) tape. The only difference is that the data is being backed up via the raw device instead of via the filesystem. A number of commercial products support this type of backup such as Legato's SnapImage or Veritas' FlashBackup.
A snapshot on the other hand, could be considered a type of mirror that is different than a traditional backup. It is on disk and appears to be another copy of your filesystem or device. However, in most cases, it is a virtual copy, that relies on the original for data.
Some vendors refer to a true mirror as a snapshot but I prefer to reserve this term for virtual mirrors provided via copy-on-write or similar mechanism.