which scenario we use High volume portal user / standard portal user? what is the difference among them?
The short summary of the difference is that High Volume Customer Portal users do not support the full salesforce sharing model, while standard customer portal users do.
What this means is that there is no way to use apex managed sharing, criteria-based sharing rules, hierachy-based sharing rules, etc. with high volume customer portal users. Instead you get a very, very stripped down "sharing" check based on lookups on your object to contact and/or account. If the account/contact your specify matches the user has the access level you specify, if not they don't.
The UI for setting these limited sharing settings up is something like this:
Also, please do read the docs! There are implications around document visibility and various other minor (unless they impact you) issues.
There is a major difference on pricing though, high volume are generally something like 25% of the price of standard customer portal users.
Whenever you have sites and need huge customer facing portal or site with authentication needed we generally prefer High Volume Portal User Licence.This can be used for huge no of portal users like 2 to 4 Million users and association with the sites this is very much preferred
While standard customer portal is whenever you have less no users and volume of users is less.
Customer Portal Manager Standard – Designed for contacts who are allowed to log in to your Salesforce Customer Portal to access customer support information. Users with a Customer Portal Manager Standard license can be associated with the Customer Portal User profile or a profile cloned and customized from the Customer Portal User profile. This enables them to view and edit data they directly own or data owned by or shared with users below them in the Customer Portal role hierarchy. Users with the Customer Portal Manager Standard license can access custom objects depending on their profile settings and can access Salesforce CRM Content if they have a Salesforce CRM Content feature license or a customized profile.
Personal groups or public groups Sharing rules Account teams, sales teams, or case teams Salesforce CRM Content workspaces - Avoid granting ownership of accounts to high-volume Customer Portal users because they can't own accounts.
You should check out the salesforce documentation.
Specifically these sections: