Hi Gaurav,
Below is available in help.sap.com
Bill of Material (BOM) Categories in Plant Maintenance - Plant Maintenance (PM) - SAP Library
Simple BOM
The technical BOM category for each newly-defined BOM is the simple BOM. This is normally sufficient for work in Plant Maintenance
Variant BOM
Combination of a number of bills of material that enables you to describe a product, or several products that have a large proportion of identical parts.
The variant BOM describes each object (for example, the product "lamp") completely. Each variant BOM contains all the components. This type of BOM is not configurable
Multi BOM
Comprises a number of bills of material, allowing you to record different combinations of materials (alternatives) for the same product.
Each alternative describes a specific material in full.
Regards
Terence