We are going to migrate 11.x to 12.x. on an enviorenment  here are some quaries:

       1.SEPM installed n Xen server, what sould be the system configuration of SEPM server for managing 2500 SEP clients ?
        

       2.SCCM(System center configurution manager) is running on the environment. What is the file extention( patch/exe or others) of client upgrading/migrating command from SEPM to clients. Will it support/work throgh SCCM?
        

       3. Replication of SEPM activated there. What is the types of SEPM replication (active-active or active-passive or else)?

       4. Is it possible to transfer SEPM from XEN server to vmware or converting XEN server to VMware ?
 

HI,

Check this artical may be help

Best practices for virtualization with Symantec Endpoint Protection 12.1, 12.1 RU1, and 12.1 RU1 MP1

Symantec Endpoint Protection 12.1 - Non-persistent Virtualization Best Practices

https://www-secure.symantec.com/connect/blogs/configuring-sep-121-virtual-environments

  1.SEPM installed n Xen server, what sould be the system configuration of SEPM server for managing 2500 SEP clients ?
        

Its always good to give extra resource if the SEPM is running on Virtual machine.

       2.SCCM(System center configurution manager) is running on the environment. What is the file extention( patch/exe or others) of client upgrading/migrating command from SEPM to clients. Will it support/work throgh SCCM?

its the exe which you use for autoupgrade.
        

       3. Replication of SEPM activated there. What is the types of SEPM replication (active-active or active-passive or else)?

SEPM is always bi directional. port 8443 needs to be open.

       4. Is it possible to transfer SEPM from XEN server to vmware or converting XEN server to VMware ?

Some recommendations for SEP and Citrix XEN:

http://www.symantec.com/business/support/index?page=content&id=TECH91070