I have a Ryzen build with a B450 Gaming Plus motherboard, and I see something called SVM mode (AMD secure virtual machine).

Is it okay to enable SVM?

No. Intel VT technology is only useful when running programs that are compatible with it, and actually use it. AFAIK, the only useful tools that can do this are sandboxes and virtual machines. Even then, enabling this technology can be a security risk in some cases.

Does SVM mode affect performance?

EDIT: apparently it is the hyperV software from windows additional features that cases the performance loss! If you turn on svm to run virtualization in Windows. The base clock will fluctuate downrange with a loss of 1.5%. i.e check with CPUz and you will see the base clock floating between 98-99.4.