Parameter is not passing from one visualforce page to another visualforce page

Question

I am new in the Salesforce coding. I have two pages. On one page I am filling the data and saving it. After click on Save button, I want to see the exact same data which I filled on last page. This is my code.

When I click on Save button first time, it stay on the same page. When I click on Save button again, it throws a message "The page you submitted was invalid for your session. Please refresh your page and try again."

---------------------------------------FIRST VF PAGE-----------------------------------------

<apex:page controller="VisitorRegistration" showHeader="false" sidebar="false">
<apex:form >         
<apex:pageBlock title="Visitor Detail Page" mode="edit">      

    <apex:pageBlockSection title="Please fill the visitor information " collapsible="true">
      <apex:inputField value="{!visitor.Name}"/>  <br/>     
        <apex:pageMessages id="showmessage">
          </apex:pageMessages><br/>                     
            <apex:inputField value="{!visitor.CountryName__c}"/><br/>             
            <apex:inputField value="{!visitor.Passport_Number__c}"/><br/>           
            <apex:inputField value="{!visitor.Covid_19_Symtomp__c}"/><br/>
            <apex:inputField value="{!visitor.SendToQuarantineCenter__c}"/> <br/>           
            <apex:inputField value="{!visitor.CanVisitorGoToHome__c}"/><br/>
            <apex:inputField value="{!visitor.Quarantine_Cetner__c}"/><br/>
            <apex:inputField value="{!visitor.PatientEntryDate__c}"/><br/>
            <apex:inputField value="{!visitor.Patient_Exit_Date__c}"/><br/>
            </apex:pageBlockSection>        
                    <apex:pageBlockButtons location="top">
            <apex:commandButton action="{!saveV }" value="Save" reRender="true" />        
            </apex:pageBlockButtons>         
            </apex:pageBlock>
        </apex:form>    
</apex:page>

-----------------------------------------APEX CODE--------------------------------------------------

public class VisitorRegistration {   
   public Visitor__c visitor{get;set;}       
   public VisitorRegistration()
   {
       visitor= new Visitor__c();              
   }

   public void getVisitorID()
   {
       Id id = apexpages.currentpage().getparameters().get('Id');
   }   

   public PageReference saveV()
   {
       If(visitor.Name==null || visitor.Name=='')
       {
           ApexPages.addmessage(new ApexPages.message(ApexPages.severity.WARNING,'Please enter value'));
        }   

       INSERT visitor;     

       pagereference pr = new pagereference('/apex/VisitorRegistrationViewPage?id='+Visitor__c.id);  
        pr.setRedirect(false);
return pr;
    }
}

---------------------------------------SECOND VF PAGE----------------------------------------------------

<apex:page controller="VisitorRegistration" showHeader="false" sidebar="false">
    <apex:form >
        <apex:pageBlock title="Visitor Detail Page">

        <apex:pageBlockSection title="Visitor Information " collapsible="true">
            <apex:outputField value="{!visitor.Name}"/>  <br/>
            <apex:outputField value="{!visitor.CountryName__c}"/><br/>             
            <apex:outputField value="{!visitor.Passport_Number__c}"/><br/>           
            <apex:outputField value="{!visitor.Covid_19_Symtomp__c}"/><br/>
            <apex:outputField value="{!visitor.SendToQuarantineCenter__c}"/> <br/>           
            <apex:outputField value="{!visitor.CanVisitorGoToHome__c}"/><br/>
            <apex:outputField value="{!visitor.Quarantine_Cetner__c}"/><br/>
            <apex:outputField value="{!visitor.PatientEntryDate__c}"/><br/>
            <apex:outputField value="{!visitor.Patient_Exit_Date__c}"/><br/>
            </apex:pageBlockSection>
        </apex:pageBlock>    
         </apex:form>
</apex:page>

Answer 1

First change your save code from

pagereference pr = new pagereference('/apex/VisitorRegistrationViewPage?id='+Visitor__c.id);  

to

pagereference pr = Page.VisitorRegistrationViewPage;
pr.getParameters().put('id',visitor.Id);

That way you're passing the Id of the record you just saved.

Also, you should change your construction to this (you do not need getVisitorId):

public VisitorRegistration()
{
   visitor= new Visitor__c();
   if (apexpages.currentpage().getparameters().containsKey('Id')) {
      Id visitorId = apexpages.currentpage().getparameters().containsKey('Id');
      List<Visitor__c> visitors = [SELECT <your fields Here> from Visitor__c WHERE Id = :visitorId LIMIT 1];       

      if (! visitors.isEmpty() {
        visitor = visitors[0];
      }
   }
}

NOTE: Since you said you were learning, I won't steer you towards using Standard Controllers, but you should research them for this specific use case.